Ok, I only know this stuff because I have spent way too many years of my life obsessing over this show. Like many other people, I have trouble watching Season 8 though and due to the gaping absence of Cindy, Michael, Betty, and even less Penny towards the end of the show (it became the Frank/Squiggy/Carmine show) it's hard to watch BECAUSE I love it so much. It should have ended with Season 7.
Anyway, here ya go!
-Frank and Edna's separation/divorce is mentioned at the end of the episode "Short on Time". You probably haven't gotten to it yet, so I won't spoil the episode for you. My own opinion of this whole thing though is that it was unnecessary. They were already able to have many episodes with the missing Betty Garrett. No need to break them up when the show only has a few episodes left!
-No, Shirley's marriage and pregnancy was NOT planned. Cindy Williams became pregnant and they had to work around it. She had just married Bill Hudson and gotten pregnant. So the reason they had Walter be in the army was so that Shirley could still be "home and pregnant" with Laverne and not with her husband. THEN when Cindy left the show, it became an excuse for Shirley to leave, but that was not the intent.
I agree I would have loved Shirley and Carmine together and Laverne/Lenny. What a sad way to end such an AMAZING and epic show.
Now apparently, they filmed an alternate "ending" at the end of Season 7 that was never aired because they thought Season 7 would be the last. They even had a wrap party and everything. I REALLY wish they would have put that ending as an extra on the DVDs.