Those are both very interesting questions. I think each could actually merit its own thread.
As others have mentioned, if D&D had not come out, a different RPG would have. It sounds like Traveller is in the running. I think that RPGs in some form was inevitable, despite the arguments that it wasn't invented earlier. The rapid increase in communications capability of the 20th century allowed people in niche hobbies such as wargames and the like to reach one another to an extent that was impossible before. I believe that collaborative effort would almost certanly have led to someone developing the RPG concept. Other things to consider are the shortened work week resulting in increased leisure time, and better manufacturing technology allowing for economies of scale to take effect with smaller batch sizes, making niche products more competitive.
Of course, without D&D, a lot of the tropes we know would likely be very different. How it would be different would depend heavily on which route of development the RPGs came from.
If Traveller were the first RPG, I think the hobby would have been slower to grow, as hard science fiction fans interested in RPGs is not as large as D&D's fanbase. I also think that Traveller would likely be overshadowed by some later sci-fi rpg that came out to better support the space opera genre popularized by Star Trek and Star Wars (unless Traveller adapted by putting out their own sourcebooks to support that type of game). I think that extending the RPG concept to other genres would have quickly followed, particularly superheroes and fantasy. Fantasy as a literary genre would probably have remained much more sword and sorcery oriented, and I do not think that it would have obtained the kind of market presence that it currently has. Many authors would not be as well known, particularly those whose works inspired D&D and who were mentioned in the D&D books as references. I am curious how Tolkien's popularity would be affected by a lack of D&D, I know it would be detrimental, but I am not informed enough to know how much.
If there were no RPGs, the time I spend on RPGs would be spent reading, possibly writing, and playing board games. I read voraciously even with RPGs taking up a lot of my time (and a lot of reading of RPG books). Without RPGs, my next most likely hobby is strategy game, so I would play more strategy board games with friends. I would probably actually sit down and try to write a novel based on the ideas I have as a creative outlet instead of using them for RPG campaigns that I run. I might end up developing a board game or card based strategy game instead, though.