Wow! What an absolute beast of a puzzle from Maskarade to celebrate this sunny Bank Holiday.
Let me say straight off the bat that I haven't finished yet, I still have a few left to do but I've reached the 'going round in circles' stage so am hoping that, after a few hours sleep and hearty breakfast, I'll have some much needed inspiration.
After reading the "Special Instructions" my first thought was "chemistry" but, after playing around with some of the themed clues, that was quickly abandoned and I realised what was going on.
If you do get stuck there are at least two long anagrams in the acrosses and at least one in the downs..
My way in to the theme was via 10d - the unadjusted answer is fairly obvious and then there are only a very limited number of pairs of letters that can be added while still keeping a proper word - the same is true with 66d. This will then give you two pairs of letters which will, hopefully, ring a bell and then, when you check to see they are "correctly positioned", you'll get all the other options.
My outstanding clues are 1a, 15a and 21a in the top left along with 69a and 78 in the bottom left. There are also a few where I'm not sure of the parsing but, like I said, after a good rest I'll hopefully have some much needed PDM's.
COD - I'm going with 29d "Pair casing the sweater girl's flat" - I had to Google why she was the "sweater girl" but it did make me smile.
If you find this as difficult as me please don't hesitate to ask for a hint - I know I always say it, but there really are so many helpful people on here willing to help.
Stay safe:-)