PCB layout for high side switch (high current)
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.