1. Corrupted Negation checks spellsave, which is a bit of downer for me. And welp, forget to mention Blood tree. My apologies. It's a neat tree for disease stacking and AoE control. Blood Fury on the other hand.. idk, I can't sustain another if i use bone shield+urh'rok(Starkeep's one)+DR+Overkill-Willful Tormenter. Another reason why I usually ignore it is because crit happens quite often in Cun 2nd Corruptor. The damage increase is great when combined with Corrosive Worm though.
How many % of monsters you think will be able to reliably save against a properly build corruptor's SP? Hint it isn't many granted some of those that are most likely to do so are very nasty. Also you don't want to crit "quite often" but 100 % often or at least close. Also if you are having max vim problems, consider dropping a worse sustain like overkill for a better sustain like blood fury and wear a max vim item if you have to, a pink item can random like 50 extra vim lol.
Shortstaves with high SP and SCrit are rare, I agree (screw you, random Telos drop), heck, shortstaves themselves are rare, that's why I said that they are meh. It's better, I think, to combine Lifedrinker (if found or vaulted) with another dagger that has blight modifier, or Shantiz, if lucky. The loss of Channel Staff and possibility of proccing blood grasp per channel is a deep loss, though. I'm kinda in dilemma..
Your argument was that reaver can duplicate DR:s crit multi somehow, a pair of extremely good shortstaves is the only way to do this since wielding an extra shortstaff is the only special EQ thing that reaver has that corruptor has not lol. If you manage find 78 % crit multi gauntlets then yes you will want to use them as corruptor too. And on insane you are very very likely to find some decent enough shortstaves during the run.
3. I often play on Insane. Lv 36 for most races is bit late, I think. The merge request for Torment is one of I anticipate the most, as it'll solve Corruptor's building problem. With that, the goodness of Light will be easier to obtain, too.
On insane you level up faster which means that you get stuff online FASTER in the sense of "how many game areas are still left" not slower. So in terms of "how many game areas are still left to explore" lvl 36 (originally you talked about lvl 20 too BTW) is earlier in the game than on lower difficulties.
4. Yes, but the bursts last only some turns before the need to Dark Portal arise, the only mean for them to escape, if Shadowflame is ignored.
During those "some turns" the monster should very much be dead. If you can't kill rares in two blasts the most after you get your crit stuff going then something is really wrong on how you build your corruptor. Also you can use movement inf just like everyone else does.