Almost all of these questions have been asked and answered many times.
MS - in older times it was dependant on a sweet spot that was unique for you. Now you want a number over 100 that is cleanly divisible into 1000 (eg: 100, 200, 250…)
You want this so you can accurately measure time for your time based stuff you are asking for.
With regards to watching procs and doing true priority it cant be done. Blizzard both explicitly prevents adding which ACT in combat from knowing this information. Unlike WeakAuras which TELLS you to do something GSE is in a sandbox that can ACT but it has very limited access to information. More information is here: FYI: Attempt at a Proc watching Macro. Result - Interface Action Blocked