A: Denoisers are the PCA mechs found in Watchpoint Alpha and Institute City. Weevils are the bipedal C-Weapon MTs that always appear alongside Helianthus machines. (And in case you missed that in-game, Helianthi are the grinder wheels).
A: Of course, just make sure to keep track of the correct order and availability of missions.
A: Knock yourself out, masochist.
A: It's the easiest way for you to check how many enemies you've killed during the mission - all costs are calculated at 90% of the maximum bonus payout, give or take a few enemies that may have been missed while testing.
A: The run was made with the idea that you should indeed do the endings in succession and not redo any missions.
A: Nope. Live with your mistakes and/or die by your mistakes.