You are able to get Isabella on good path, but it's easier on neutral path.
For Cerea and Yisnna scene on latest version, you need to have had a previous scene with Yisnna in the library and you need to be on Cerea "Blackmail/rent" path, note that if you are on the "Blackmail/rent" path, you will miss the "Love path" scenes... Or something like that... 😅
sounds like you took the same route i did. i am not really good at finding triggers. still have idea where isabelle's first trigger is. anyway im on good path and i have not got the threesome either. Just retried it with mida there and just rent no blackmail no good. so you might be right about Mida being as the problem. if you've started the blackmail route got temple and both library scenes plus rent scene.