Crossfire (and SLI, for that matter) are usually something you'll want to consider for a rather high-end build. I was toying with the idea of just throwing a second 680 into my PC instead of upgrading to a 980, but at the end of the day, you're still paying for old tech. If you're shelling out, I'd say invest a few more bucks and get yourself something that's gonna last a little.
Eh, that stuff has been going back and forth for quite some time now. There's been an "optimisation war" for years, AMD getting their logos onto some games, NVIDIA on others. AMD released their TressFX and Tomb Raider ran like crap on anything made by NVIDIA at first (in 2013). Then came GameWorks, half a year later - seemed to me like NVIDIA did that one out of spite, at the time

. Now there's Mantle and G-Sync, too... They've both been playing the same game, as far as I am concerned. Can't blame them for trying to get a leg up on their competition, either.
Maybe on the entry level GPUs, never looked into those. As far as gaming cards are concerned, no. Lots of benchmarking going on these days. And that doesn't rely on games with lopsided optimisation. I've yet to see AMD touch a GTX970 on neutral grounds, let alone a 980, Titan or 980ti.
As far as price goes, a 970 costs just as much as a R9 290X. The 970 pulls ahead in pretty much every benchmark I've seen, though. And, yeah, the games I've cared about are optimized for NVIDIA, too, so there's that. Believe me, if I saw a point in going AMD, I would've done so.
Meh, never had issues with either brand and don't know anyone personally who's had a card go belly up. And my old 680 had to take a lot of abuse (overclocked it a
wee bit past the recommended limits

); only thing I've got first hand experience with (and heard a lot about) is how hot AMD's GPUs and CPUs are running. As I said, never ran into trouble with either company, so that's the only thing I can go by.