I fixed the assert and dropped first iter compat-mode token bugs one-by-one by writing narrow unittests that fail and code that makes them pass. I am now working on the '\' continuation issue. That is the subject of #9974, which has a nearly identical patch. Where it differs from yours, I will choose on the basis of tests. Any further discussion of this bug should be on that issue.

I appreciate the warning that the full mode is undertested, so I need to be concerned about breaking untested functionality that works. That was not so much a concern with the first two issues.