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Any automotive vacuum gauge is fine, measuring from 0 to 20" Hg is adequate. 8" is the crucial point, most are very close in this part of the range. Most will also measure positive pressure, this is fine.

Could be. The best test is a vacuum gauge on the top hose. The vacuum should drop uniformly as the throttle is pressed. Nearly impossible to clean/repair, replacements are $65+

There are a few different sizes from the vacuum pump to the bottom fitting. Also a tee and cap. Check these carefully, there must be at least 18" Hg at the modulator. Any leak will be a problem. I would pull the vacuum lines off and get exact replacements from your local auto parts...