BaalHabooze, I think the answer to your question on why we dont celebrate a 9th day because of sfeika diyoma is that Chanukah isn’t a yom tov mentioned in the Torah-it is a yom tov midivrei sofrim, as the Mishna Berurah says it, and I don’t believe we ever keep an extra day for something Midirabonon.
The question I have is why Yom Kippur isn’t 2 days long!!?? Why don’t we say sfeika diyoma? Maybe a possible answer is the same answer as in the SHulchan Aruch? (I think) that we don’ want to go two days without the possibility of burying a mes-the same reason why Yom Kippur is never on Friday or Sunday, though that itself may be based more on Hoshanah Rabbah not falling on Shabbos, an additional question now that I think about it.