1.- That's funny but ya, that's something that should be fixed.
2.- I don't think that's a bug, enemies who get rage per shield activation will, well, do exactly that (take is as a downside of spamming shields), now, it makes sense that every shield only get to use positive effects once per turn, otherwise you could get a ridiculous amount of haste, given you have 1 haste per shield, or in the case of cleavers (rage), they would be WAY stronger than they already are.
3.- It makes sense that you can only kill the Elder Mage in one turn if it's a literal one shot because it is a very fragile boss, and it's supposed to be a boss, not an enemy with slightly more HP. But i agree on the fact that vampirism shouldn't trigger the dodge effect with that boss, i have never been in that situation but hitting the boss for the 1, 2, 3 vampirism damage for it to dodge your BIG damage sounds horrible.
I hope you keep enjoying the game! ^^
Messing around with curses was quite fun, first time testing haste in a shield, i'd love to have negative armor but it'd take way too long with just 1 haste per energy; Orksork Axe came in clutch with the P2W and saved me multiple times.