Electronic – Why do surface mount mosfets have the heat sink pad on drain rather than source? (N channel)

To directly answer your first question: No, not even close. A little more copper around a surface mount part isn't going to get rid of 11W of heat. No way.

One answer may be to parallel multiple FETs. Not only does that cut the total dissipation by the number of parts, but the dissipation on each FET is reduced by the square of the number of parts. So if one FET dissipates 10W, then two parallel FETs would dissipate 5W total, and each FET would dissipate only 2.5W.

That's in theory. In practise they won't share the load exactly equally, so you have to design for a bit worse than that per FET. The good thing about paralleling FETs is that they have a positive temperature coefficient. The Rdson goes up with temperature. This helps them ballance somewhat and prevents runaway of a single part, like could be the case with bipolar transistors.

Ultimately you have to decide what you really want. Switching 40A is going to make some heat. One way or the other you're going to have to deal with that. You can tell us you're space contrained all you want, but ultimately the physics will dictate a certain amount of space, surface area, forced cooling, or whatever. It may not be possible to meet all the constraints. Not all combinations of small size, high current, and low cost are possible.

If you're not using an insulation kit then the only option is tie it to the same net as the tab on the transistor.

If you are insulating then connecting the heatsink to gnd may add to much capacitance to the circuit that's on the tab. I'm thinking of the drain of a switching FET running at MHz as an example. But, by the same token, connecting it to the tab may produce a bigger radiator of interference to sensitive circuits located close by. Leaving it open circuit may sometimes be the best compromise.

I can't envisage a reason for using a resistor to ground it but, maybe there might be a subtle benefit when insulating it instead of leaving it open circuit.