Since season one took place 5 Years Before Yavin and this will span 4 years, the one year time jump every three episodes means this will likely end where Rogue One begins.
I'll spoiler tag this just in case but it's related to your above i thought it had been stated somewhere that the end of s2 will literally be the beginning of Rogue One and you can start that movie and it'll feel like a direct continuation. I expect an almost seamless transition from the end of s2 into Rogue One. I don't know the source but I recall it being out there from an official source
I am really hoping we get a Rebels crossover since the 4 year timeline where season 2 takes place lines up to the bulk of that series. Mon Mothma defecting from the Imperial Senate and publicly declaring the Rebel Alliance happens in season 3 of Rebels, and that is such a pivotal moment for that character and the overall canon that Andor has to depict it's own perspective on it. It is what that character's entire arc is leading up to.
I don’t know if we have to specifically see that moment honestly. There’s other ways it could play out like Mon Mothma confronting her husband about it or something like that. I’ve seen it in Rebels, I don’t have to see it recreated in live action.
I do think you'll hear it from Cassian's perspective. I do think it has to be addressed and shown one way or another. It's a pivotal moment for the character.