Try the following
I resolved this issue with something similar to the steps you've provided (moved acountDataDB.package, UserSetting.ini, temporarily removed mods, deleted localthumcache.package, repaired game). And I was able to get Research Ambrosia to pop up (on my old save game even) and completed that series of quests before restoring my files and was able to keep the progress and move onto the next week of quests.
To provide additional information, I think that what went wrong in my game was that I was initially able to start the process of researching, but then my sim got interrupted by the Paranormal Freelancer Career Gig. When I got back from the gig, I wasn't able to continue researching. So it seems like it may be possible that at least one of the ways that this occurs is if a sim starts doing research but is unable to complete the research in one go (and maybe traveling off lot could contribute as well). The option gets removed and there's no way to continue or restart.
I do use mods, so it may be possible that it might have been caused by a conflict, but I figured the information may be useful to help debug and maybe prevent some people from encountering this problem.
