Took me some time to get this hammered down so hopefully this will help:
1a. If DmgProc activates on one skill, then you begin another skill, then the DmgProc ends. You get (next to) nothing as whatever part of the previous skill was active was the DmgProc got the boost.
1b. See your point, and the duration of the countdown really doesn’t work this way. Believe it’s more meant to act as a ceiling on channeling skills so that they cannot just go on forever with an active DmgProc. Also is meant to reward skills with long animations and multiple hits (Psylocke’s 5, Wanda’s 3, DP’s 4, etc.).
Once the DmgProc becomes active it will boost the dmg of the remaining hits of the skill. It is not retroactive to the beginning of the skill. Thus, better to have the DmgProc activate towards the beginning of any skill.
This is where muscle memory and SCD meet. After the 7 second cooldown you then have a 10% chance of reactivating the DmgProc. So you either want to leave something out of your rotation? Or have something run a bit long to get you back on track where the Proc hopefully activates at the beginning of the intended skill.
In my experience you need to activate your desired proc skill then hope like hell it activates the DmgProc. Being that it is a “10% chance” it is not a perfect science.
Do you have Cable? He has a very user friendly skill set that is as close as possible to a “sure thing” dmg proc activation. Play around with it in ABX to really get a feel for it. I’m a year in and still find it challenging to hit Wanda’s procs with accuracy. Today is Luna Snows day. If you have she’s a good one to play around with. Try and activate the dmg proc on her 3 skill.
Hope this helps.