can I make the ping out from n0 go through n1 -> n2 -> n4 -> n5 and return on n5 -> n4 -> n3 -> n1 -> n0?
Yes, you can.
As per the information in your post, n0's IP address is 10.1.5.2. Therefore to alter the return path you will need to change the route for 10.1.5.0 on n4.
Currently it looks like this:
10.1.5.0/24 via 10.1.6.2 dev eth2 What you will need to do is change it to this:
10.1.5.0/24 via 10.1.1.2 dev eth4