For the first case, not getting on your connecting flight will cancel the rest of your itinerary.

The same can be said of your second proposal. If you don't show up in LGA, then your entire ticket is cancelled.

Buying nested tickets, which is sort of the other thing you propose is definitely something the airlines don't like. For example, it may be cheaper for you to buy one BUF-CLE and one CLE-BUF ticket, both with Sat. night stays. You may get away with it if you don't input your frequent flyer account into the itineraries, but if you do use a loyalty program they will pick up on it in a second.

Airlines call this GDS abuse and they are really starting to crack down on this.