TRIM is very nice but the drive will still lose some performance as you fill it with data (I'm not talking about writing temporary data or such but about data that's kept on it). But yes SDDs performance does not degrade and if you format it you get 100% of your performance back.
Nope, you can still use the rest of the drive, as long as it's not being accessed while playing. Perfect for keeping other games or data. My gaming HDD is like 90% full (but partitioned) and does not lose any performance. But it only contains games and I don't play several games at the same time though, then a SSD would be much better.
I have plenty of 5+ years old HDDs, if you buy the right models from the right brand and take care of them (no shocks, no overheating) HDDs are in my experience very reliable, which is to be expected from an old and mature technology. SSDs may turn out to be more reliable (they SHOULD) but so far they are plagued with firmware issues and the like and they have not been on the market for long enough to properly assess their reliability over time. But surveys have shown that as of yet they are not more reliable than HDDs overall (plenty of bad series and whatnot).