HIMYM can get away with those "sitcom contrivances" better than most because the whole thing is a story told in flashback by an unreliable narrator. I'm inclined to think that Future Ted isn't remembering most of the events correctly, nor is he accurately describing the amount of time they spent at MacLarens. When it comes down to it, I doubt his tale of Barney's sexual history, for one thing, or general awesomeness is anywhere near accurate. The reality of HIMYM is probably far different than what we, the audience, sees through Future Ted's tales.
And is this whole story of how he met their mother being told in one sitting?