1- No. don't drain your engine oil to adjust valve lash but do put a small container under the cover to catch the 2-3 teaspoons of oil that will come out of the loosened OHV cover.
2- Correct valve lash means two things. Each valve in the pair should be as close to it's mate as you can get it. This seems to improve engine smoothness. So, adjust the intake valves on one side of the motor so that they are equal. Same for the exhaust. As for actual valve lash, not as important as equal. I used to visit with an experienced BMW tech where I bought my GS. He said he had some customers who preferred the valve lash to be a bit tight for added high RPM performance. If requested, he adjusted oilheads to one half the BMW lash spec for both intake and exhaust without consequences and this was in Houston TX summer heat. So, unless your exhaust valves are have no clearance or you are experiencing intake backfiring, I would not be concerned about discovering tight valve lash.
3- Me? I run my valves loose deliberately in order to improve starting and provide smooth engine operation at the lower-mid RPM range. I rarely explore red line on my '01 boxer. There just isn't any reason to go there unless you like high speed combustion and a slight loss of power. Last time I was at red line, I had forgotten to shift from 4th to 5th when I was racing a cage. I actually saw just over 7000RPM on the tacho...never done that before. I was not impressed as I felt the power fall off.
4- All valve lash adjustments should be made with the engine stone cold.