It is clear to me that any fork of dwm must also be licensed under an MIT/X-compatible license

That is not, in fact, clear. Firstly, if you read this question you'll see that there is quite a body of opinion that says it's perfectly acceptable to change the licence conditions on code if it was shared with you on terms that do not forbid that. The copyleft free licences require copies and derivatives to be distributed under the same licence (see eg GPLv3 ss 4 and 5c, GPLv2 ss 1 and 2b) but the non-copyleft licences simply don't have similar language - they require only the distribution of specified text. Certainly, it's known that derivatives of BSD-licensed code are used in commercial products distributed under proprietary licences.

Secondly, you don't address the issue that it's very hard to write a patch for a piece of software without carefully examining said software, and copying the interfaces so that your patch can cleanly replace or supplement existing code. Those are very strong arguments that the patch is a derivative in copyright terms, whether or not it ships with the original software. What that implies for distribution depends on the original licence.

Those things said, let's examine your specific questions.

Suppose Alice writes a patch of dwm, and explicitly licenses her changes under a restrictive copyright license. Alice could distribute her changes to whoever she likes, but could not distribute a fork of dwm which included those changes

Assuming that dwm is licensed under a non-copyleft free license (such as MIT), Alice is completely free to distribute such a fork. It is, as I have said, a common-enough commercial practice.

Suppose Alice writes a patch [which she intended to be a patch of dwm] and does not explicitly license it at all.

As has been said here before, if code is distributed with no specific licence terms, and it's not required to be free by eg being a derivative of a third-party strong-copyleft work, you can make no assumptions about what you can do with it. The patch is proprietary, and thus unusable by the free software community.

Suppose Alice writes a patch, and explicitly licenses it under a license like the GPL... Are forked versions forbidden in practice, just like in situation 1

No, that would be fine. Such combinations are permitted provided the original licence is compatible with the GPL.

It is clear to me that any fork of dwm must also be licensed under an MIT/X-compatible license

That is not, in fact, clear. Firstly, if you read this question you'll see that there is quite a body of opinion that says it's perfectly acceptable to change the licence conditions on code if it was shared with you on terms that do not forbid that. The copyleft free licences require copies and derivatives to be distributed under the same licence (see eg GPLv3 ss 4 and 5c, GPLv2 ss 1 and 2b) but the non-copyleft free licences simply don't have similar language - they require only the distribution of specified text. Certainly, it's known that derivatives of BSD-licensed code are used in commercial products distributed under proprietary licences.

Secondly, you don't address the issue that it's very hard to write a patch for a piece of software without carefully examining said software, and copying the interfaces so that your patch can cleanly replace or supplement existing code. Those are very strong arguments that the patch is a derivative in copyright terms, whether or not it ships with the original software. What that implies for distribution depends on the original licence.

Those things said, let's examine your specific questions.

Suppose Alice writes a patch of dwm, and explicitly licenses her changes under a restrictive copyright license. Alice could distribute her changes to whoever she likes, but could not distribute a fork of dwm which included those changes

Assuming that dwm is licensed under a non-copyleft free license (such as MIT), Alice is completely free to distribute such a fork. It is, as I have said, a common-enough commercial practice.

Suppose Alice writes a patch [which she intended to be a patch of dwm] and does not explicitly license it at all.

As has been said here before, if code is distributed with no specific licence terms, and it's not required to be free by eg being a derivative of a third-party strong-copyleft work, you can make no assumptions about what you can do with it. The patch is proprietary, and thus unusable by the free software community.

Suppose Alice writes a patch, and explicitly licenses it under a license like the GPL... Are forked versions forbidden in practice, just like in situation 1

No, that would be fine. Such combinations are permitted provided the original licence is compatible with the GPL.

It is clear to me that any fork of dwm must also be licensed under an MIT/X-compatible license

That is not, in fact, clear. Firstly, if you read this question you'll see that there is quite a body of opinion that says it's perfectly acceptable to change the licence conditions on code if it was shared with you on terms that do not forbid that. The copyleft free licences require copies and derivatives to be distributed under the same licence (see eg GPLv3 ss 4 and 5c, GPLv2 ss 1 and 2b) but the non-copyleft licences simply don't have similar language - they require only the distribution of specified text. Certainly, it's known that derivatives of BSD-licensed code are used in commercial products distributed under proprietary licences.

Secondly, you don't address the issue that it's very hard to write a patch for a piece of software without carefully examining said software, and copying the interfaces so that your patch can cleanly replace or supplement existing code. Those are very strong arguments that the patch is a derivative in copyright terms, whether or not it ships with the original software. What that implies for distribution depends on the original licence.

Those things said, let's examine your specific questions.

Suppose Alice writes a patch of dwm, and explicitly licenses her changes under a restrictive copyright license. Alice could distribute her changes to whoever she likes, but could not distribute a fork of dwm which included those changes

Assuming that dwm is licensed under a non-copyleft free license (such as MIT), Alice is completely free to distribute such a fork. It is, as I have said, a common-enough commercial practice.

Suppose Alice writes a patch [which she intended to be a patch of dwm] and does not explicitly license it at all.

As has been said here before, if code is distributed with no specific licence terms, you can make no assumptions about what you can do with it. The patch is proprietary, and thus unusable by the free software community.

Suppose Alice writes a patch, and explicitly licenses it under a license like the GPL... Are forked versions forbidden in practice, just like in situation 1

No, that would be fine. Such combinations are permitted provided the original licence is compatible with the GPL.

It is clear to me that any fork of dwm must also be licensed under an MIT/X-compatible license

That is not, in fact, clear. Firstly, if you read this question you'll see that there is quite a body of opinion that says it's perfectly acceptable to change the licence conditions on code if it was shared with you on terms that do not forbid that. The copyleft free licences require copies and derivatives to be distributed under the same licence (see eg GPLv3 ss 4 and 5c, GPLv2 ss 1 and 2b) but the non-copyleft licences simply don't have similar language - they require only the distribution of specified text. Certainly, it's known that derivatives of BSD-licensed code are used in commercial products distributed under proprietary licences.

Secondly, you don't address the issue that it's very hard to write a patch for a piece of software without carefully examining said software, and copying the interfaces so that your patch can cleanly replace or supplement existing code. Those are very strong arguments that the patch is a derivative in copyright terms, whether or not it ships with the original software. What that implies for distribution depends on the original licence.

Those things said, let's examine your specific questions.

Suppose Alice writes a patch of dwm, and explicitly licenses her changes under a restrictive copyright license. Alice could distribute her changes to whoever she likes, but could not distribute a fork of dwm which included those changes

Assuming that dwm is licensed under a non-copyleft free license (such as MIT), Alice is completely free to distribute such a fork. It is, as I have said, a common-enough commercial practice.

Suppose Alice writes a patch [which she intended to be a patch of dwm] and does not explicitly license it at all.

As has been said here before, if code is distributed with no specific licence terms, and it's not required to be free by eg being a derivative of a third-party strong-copyleft work, you can make no assumptions about what you can do with it. The patch is proprietary, and thus unusable by the free software community.

Suppose Alice writes a patch, and explicitly licenses it under a license like the GPL... Are forked versions forbidden in practice, just like in situation 1

No, that would be fine. Such combinations are permitted provided the original licence is compatible with the GPL.