Pad and soldermask dimensions in PCB design

If you want affordable boards, forget about blind vias, via in pad, and filled vias. This is a good presentation on BGA routing, albeit for very high-density boards, but the basic principles will be the same for less demanding layouts.

SMD pads vs NSMD pads is something you need to ask the company doing your BGA assembly about. The latter seems to be preferred. Some chip manufacturers have recommendations, as well.

If you have questions, this forum is very useful. You can also learn a lot by reading the various posts.

If you're interested in solderability and manufacturability, then you should follow the recommended pad layout. I've never had a manufacturer's recommended pad layout give me grief, although I have run across a few vendors whose dimensioning requires a fair bit of headscratching and pencil-and-paper chicken-scratching in order to figure out offsets and spacing.

You are making some bad assumptions in your calculations though. No, pins are not often centered on the pads "lengthwise" but they are often centered widthwise. Y1 and C1 would most certainly be given by the manufacturer. The recommended land pattern will (in my experience) give more space for the pin on the "outside" of the pad and less underneath it. My guess is that that gives a good shape to the solder connection. You won't have anything in terms of heat dissipation unless there are a lot of grounds or you have a ground pad underneath. In the case of a lot of ground pins, you'll want to give them a lot of copper fairly quickly, but you'll want to connect the pad to the copper with thermals or you'll have soldering problems.

I wouldn't worry about minimizing board space, especially if you're not building a million of these. The half a millimeter you might save by shaving Y1 a little isn't worth it.