The point change is not the same every time because it depends on the mean and deviation of both players' ratings. I'm not sure what you mean with proportionate. There is no guarantee that the sum of the ratings stays the same. For players with settled rating a win gives about 8 to 14 points. A draw maybe 2-3? I can't think of a game with a draw of the top of my head, but you can check for yourself in the client if you click on a replay in the client and then press the "show rating change" button.
If you continuously draw against the same player (and don't play against others) your mean rating will eventually become equal.
The speed at which is happens is in theory an adjustable Trueskill parameter, but only in general for all rating changes, not just draws. So if you reduce the speed, you will also reduce the points you gain/loose from winning/loosing. The parameter controls the "volatility" of the ratings. In theory you could measure which parameter value would make Trueskill predict the outcome of a match correctly most often, but that would be quite bit of work and i don't think the current value is too bad.
(I think there is a different parameter that is supposed to tell Trueskill system how likely draws are in general but i am not sure what changing that would do.)