Spoilers for the Light Novel
1.He'd be significantly weaker and wouldn't have evolved or done most things without Great sage/Raphael.
2.Great sage was essentially the guide so without it he'd be still weak, he wouldn't have grown so fast and would have to learn at everyone else's pace.
3.Well it would be relatively impossible considering I may be wrong but veldora and Rimuru are connected and for either to die permanently you'd have to kill both at the same time and Hinata isn't beating veldora, but in the assumption that she did kill rimuru Chloe would have reverted time again, tempest would go berserk and ultimately be defeated for being reckless now this is Pre Diablo, The black numbers, his awakening to demon lord but this really depends on which fight, pre demon lord evolution fight tempest is destroyed and Veldora rampages, milim rampages, and the world is effectively cooked. Post demon lord evolution is so much worse with Diablo killing everything in a rage, Veldora and milim absolutely rampaging and in the LN there's already an explanation on what would happen but to put in short terms the world is finished with no if's, and's or but's.
