Dh's grandmother was bipolar & his cousin is. I have not researched it & this is just anecdotal but it fits your "skips a generation" research. Dh's cousin was dx. in his early 20s, FWIW.
We were not so concerned that we opted for donor sperm, but OTOH, I would not go with a donor with a mental illness that. I don't dwell on it but I do hope our 2 children are not affected, it goes without saying.
My 1st DE cycle was a miserable failure with a proven donor & my two children are via an unproven donor, so unlike some, I don't put much extra stock in proven vs. unproven, esp. if you trust your clinic.