A blind 20% bump in pressure is way too big to go and just recommend to everyone. The reason for the jump from 58 to 75 is that 58 is a commonly used torque value for cast iron heads. 75 is used in most things aluminum (your wheels, for example). People far more educated on matter than you and I came up with these values, their warranties are based on these values (yes, they will be able to tell). The head isn't as beefy as people like to think and the bolts and studs aren't immortal. Any, I'll reiterate that the value also depends on the lubricant used... if you're using 30 weight oil, your about right on... moly... well, whoops.
You can't really drill fasteners without compromising their elasticity. There's no way you're going to keep them cool enough for starters. And it has nothing to do with creating a vent, it's for measuring stretch and calculating the real clamping force.
The dowels are not considered reusable actually... no reason they shouldn't be except that they're tapped into the block and getting them back out requires either pounding then in the rest of the way so they fall into the coolant passages or mauling the living hell out of them to pull them out the top. This came from a Toyota tech, there's nothing in the TSRM on the matter as they don't expect you to remove them at all.