If a game continues indefinitely, repetition is guaranteed to occur.
If repetition occurs in a game where both sides are using best play, the continuation will follow the exact same line as the first time the position was reached, eventually resulting in a third repetition....
We can split hairs about whether the rules dictate that the game be stopped at a certain point, or the repetition be left to occur indefinitely, but it's irrelevant as the game is drawn in either case.
The idea that with best play chess cannot be infinitely long rests on a logical fallacy. How can a player use judgment to determine that a position is drawn, when "best play" is only defined in computer terms of being able to compute every possible variation to its end? The argument begs the question. For "best play" to be determinable, it depends on a finite game.
Once again, that is incorrect. If a position is reached that could be duplicated indefinitely, then that position is a draw. By the current rules, there are protocols that must be followed in order to claim the draw, but on a mathematical level, a repetition never creates winning conditions for either side.
It can be easily shown that Chess moves are infinite, but the game itself is not transfinite. If a game exists with N moves, then a game must exist with N+1 moves, i.e. all the way up to infinity. However, at some point, one of 3 conditions will be met, 1) white wins, 2) black wins, 3) draw. There is an upper bound at which this condition will be satisfied for every game. So yes, it is possible to have a game where a position is repeated indefinitely, but one of the conditions (draw) will be satisfied. The fact that an infinite number of positions exists does not change Chess's solvability.
As far as judging indeterminate positions as a draw, again it can be shown that with best play on both sides, eventually a position will be reached where neither side computes an advantage, no matter how far the depth. This function is further simplified by the fact that as the number of pieces goes down, there are less legal positions to consider. In fact, all games of 6 pieces or less have been solved by endgame tablebases.
http://en.wikipedia.org/wiki/Endgame_tablebase
I know that you have a strong conviction that Chess is unsolvable, but my (and others) conviction is that it is. While I don't expect you to change your mind about it, there is an overwhelming amount of evidence pointing to the eventuality, Moore's Law notwithstanding. As I have pointed out before, it is still possible that other systems of computation have yet to be discovered, as human thought, for example, is clearly different than binary computing.