May I make a suggestion for now ?
If I understand correctly you can boot using an older kernel and somehow get to a working situation, although not ideal.
If that is the case, then install one of the later kernels and test it again.
Things have changed with the partition layout.
Installing a new kernel will allow it to build its initrd as things are at present, and thus may resolve the problem you met with previously.
If the kernel still shows a problem which cannot be over come then it can be uninstalled again.
Of course keep the older working kernel until you are very sure all is OK with the newer one (if at all).
Having just re-read the following, maybe I have the wrong impression of the state of things .....
I hope to be able to get in to the other partitions and re-install the old kernel while leaving things otherwise as they are, to further test whether they are the source of my trouble.