Well, again, Im not entirely clear on just how much of having a riff from a tune played in a midi you can get away with. There is a point where "sounds like" and "ripped off" converge.
As far as scrolling lyrics, they are indeed under copyright, and must be licensed.

Commercials do indeed pay to have music licensed for use. Length has nothing to do with it, it doesnt matter how long a clip is, if it can be identified as from their work(and you would be surprised), then your ass is grass. :)
See my above rule of thumb:P