The pressure depends on the radiator cap. Usually there is a 14lb cap, but you can fit a 7lb cap to keep stress off the cooling system. I think it's a good idea to lower the pressure, as long as you are not running the near-boiling thermostat.
Yes, the holes are to heat up intake manifold & carburetor base.
Most newer heads have holes right there, but not all of them. Take a look:
A12A Cylinder HeadI haven't had any problems with the Solder Seal brand of stop leak. It fixes small pinhole leaks, but not gasket leaks. Use as directed and all that. Don't bother using it to fix holes bigger than what the directions indicate or you just might have problems.