Can I combine multiple 2-Clause BSD LICENSE files into one?

The BSD 2-clause license is really permissive. The only condition which applies to distribution of source code is this one:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

The three parts of the license (the copyright notice, the list of conditions and the disclaimer) must all be retained but they are listed separately, not as a block, so there really is no restriction which would prevent you from "merging" several BSD-2-clause LICENSE files into one.

Thus, all the solutions you propose are fine. I would personally go with the last one because it is the simpler. (Note that "All Rights Reserved" is traditionally part of BSD licenses but serves no special purpose: https://opensource.stackexchange.com/a/4403/5858.)

However, I would bet that if someone borrowed your code (and not the dependency), they would not understand that they can remove the copyright notice of the other dev. That would not really be a problem however (a factually incorrect copyright claim does not void other, factually correct, copyright claims). If you want to be sure that they understand which copyright notice applies to which part of the work, just keep the original notice in the subfolder and do not reproduce it at the root. The license does not force you to do that. (I would then add a note in the README telling about the dependencies but this is just good manners.)