I found the following additional resource.
2of12inf.txt from the 12dicts package.
It successfully contains all of BUNNY TAPIR CAT CATS MOUSE.
It successfully omits all of EURYPTERID AALII SSGT ATONCE CYRUS CATTED CATTING.
I am not yet aware that this list has any disadvantages, for my purpose!
First of all,
At this point the grid looks, if I haven't miscalculated, like this:
Credit where due: Deusovi started on this before I did and worked faster than I did, and had he not run into errors in the original puzzle he'd doubtless have finished long before I did. (I haven't looked at anything in his answer, though, beyond checking whether the ...
Partial Answer (outdated by changes to the puzzle)
The trick to this puzzle is that
The first step is to look at
Now that four of the variables are determined,
This seems to be a contradiction, but unfortunately I can't continue working on this at the moment.