The name of a footprint that is designed to let you cut the trace between two nets

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.

I'm curious how you derived your power dissipation numbers. Looking at the data sheet it looks like 10ams 200 mW (12 degree temperature rise), 30 amps, 2.5W with a 90 degree temperature rise (given the Rthja of 40 degrees/W which seems to be true even if you have 6 cm^2 of PCB area).

That said, if you want to pull a lot of heat out of your FETs you can have a .250" plated through hole drilled under them and then use a copper slug which extends up through the hole and contacts the back of the package. you could also glue a heat sink to the top but it is not as effective trying to conduct through the case.

To your layout questions, it looks like a 6mil trace for all of the source leads. That would be a poor choice at 30A, by comparison look inside a 30A fuse :-) What it means is that you'll get some warming on that trace. What ever trace width you pick, do the calculation at your chosen copper level and use current squared x resistance to compute how many watts that trace will dissipate.

You don't need a all the vias you've got on the pad. 5 would be sufficient to thermally connect top to bottom. I've seen people just use one, but you rely heavily on the plate though of the hole in that case.