The scar on his chest is where Goku punched him, where otherwise he's immune to harm, hence why I say it's the cause of his first loss as well as his second leading to his death, as his green guts somehow start exploding out of it in the sun.  The borrowed power was just necessary to even harm him at all, what with Goku's power dwindling.

And the whole Saiyans surviving space has evidence of them surviving just in plain old space like Bardock matching Frieza, yeah, but also them dying as with Blue Vegeta surviving Frieza blowing up Earth but not lasting without oxygen in Resurrection F.  Sure wasn't near empty on power Frieza overpowering him with that blast or just the planet exploding.  It's up to the situation at hand as to if they get to fly around in space or not.  I don't recall where the theory came from, but I was always partial to the idea that they just create oxygen with their ki similar to Green Lantern, since ki does anything else they want.  It's pretty much the Force really.

Timeline-wise Broly took place in the wait after Cell became complete and announced his Cell Games but they had not started yet.  Biggest hole in why it had to be non-canon was just that everyone was relaxing at the time, instead of just Goku and Gohan learning to master the default Super Saiyan form by chilling out, if I recall correctly.  So being Cell could be obliterated by a full power Super Saiyan and Broly could not, lets presume he's around SSJ2-ish levels in power when just entering the Legendary state, yeah?  Would match to his credentials of destroying part of the galaxy for fun.  It seems to me he way outclasses Hulk for way too long before Hulk gets a chance to gain an even footing in power, even if Broly toys with him.

I never cared for Doomsday exceedingly, so fill me in where I'm wrong.  So the concept behind him that I get is that while he suffers many "deaths", it's in the way of Wolverine in that he can heal back from anything that seems to put him down, but more broken in special abilities is that he gains an immunity to that form of death.  When punched to death, he becomes immune to physical forces.  When heat blasted, he no longer is affected by heat sources.  Maybe Hulk can accidentally twist that to his advantage by super clapping him to death, causing Doomsday to go deaf but beyond that Hulk's means of destruction are fairly limited, just ridiculous in scope of power behind them.  So splat Doomsday a few times, he becomes immune to anything Hulk can do then it no longer matters how strong Hulk gets when Doomsday becomes an immovable object.  How then Doomsday would kill him though I dunno, does his strength grow too?

As far as that DBX match is concerned, Yu Narukami can kill Gods. Can Jotaro?

He was at his strongest in part 3, and since then had to ge nerfed to make the others look better.
Time freezing and slight was a bit op when you think about it