10/10 up until you got blasted by yous knows whos!
I think I might have bought that better. Still ultimately a "crapshoot" ending, but not so ambivalent. "We'll try it on our own, [censored] off Reapers" or "We'll throw our lot in with you and pray we can stop Dark matter"... dunno, might have felt more solid.
No, none of those DLC are critical. Kasumi and Zaeed will show up on side missions, if you've had them in ME2 and they survived, and Liara will talk about being the Shadow Broker whether you did the DLC or not.
Arrival isn't necessary - just sort of sets up for the prologue of ME3, why Shep is on Earth, and releived of duty when the game starts.
I actually liked the From Ashes DLC. ymmv, but it nets you another crew member, which is fine by me. Had some interesting insights.