1 - Slims run much cooler, I'll let someone else chime in on play time, but I wouldn't play for over 3 hours without taking a break, at the very least to go for a walk, don't need you dying from health issues related to not moving!
2 - Yes, they do
3 - No, every update is inclusive of the old ones, so you could in theory, go from 1.00 right to 3.65 without any issue (not on a slim, it started around ~2.70 but you get the idea)
I've played my Slim for 8+ hours (much to the detriment of my health as CJPC noted ) without any issues. They don't get hot like the Fats do, so it will last longer than any play session you can endure.
a few years back i played my fat 40 gig for 36+hours straight (after using amphetamines)and all was fine.. it doesn't overheat, touch wood, and since i've put a 500gbyte hdd it runs even cooler never had extra fan or mod of any kind... probably cooler than my slim 320gbyte.. i play nearly every day for between 2 to 10 hours (being single sucks)
oh and i move to go to the fridge and to the toilet and to drive to the chemist everyday.. LOL