I'm sure you can start watching now (I started watched Buffy as S4->S7, S1->S3), but I have to believe it's better to start at the beginning and work your way up. Otherwise, you'll know a lot about where the characters are going when you watch those earlier episodes, and you won't know where they came from this season if you don't watch the earlier episodes first. You'll still get a lot of enjoyment watching out of order, but you'd get more enjoyment watching in order.
Living in the house your ex paid (and pays) for rent free with your new man in asking a lot. It'd be different if it were just her.
The new advertising agency has a long way to go, but has already come a long way since we left off.
Don continues to have only a passing relationship with the truth, and he's one of those people who gets rewarded for lying (he's good at it). Then again, some would say that's the essence of a successful ad man.
For awhile there it was difficult for me to remember how last season left off. It almost looked like a flashback because all the main characters from the old agency were at the new one, even the original bosses.