Sorry - me again (and then I'll shut up for a bit). I did look for an alternative placing for the 5-letter thematic, but assuming we're talking about replacing an answer rather than a string of letters (consistent with the other two) , all other possible locations lead to non-words in the crossers.
I am completely baffled by this. From the across extras I have a 5,3,2,10 phrase which becomes a 7,8,5 phrase from the down thematic (8,6 ?), then I am lost.
Feeling stupid here as I can’t get the endgame. I have googled my down extras and have found two others which somehow have made 15 and 36 across catch my eye. I assume I have to omit some letters from the original quotation which will then point me in the right direction in the grid.
There are certainly laws for all three though one is physics. I have tried omitting middle letters and get gibberish and am not sure whether it should be the middle letter or all middle letters. I’ve also tried taking the individual letters in MIDDLE away.
I think the PDM has just hit me like a ton of bricks - my 8,7,5 from applying the the down answers letters replace 3 thematics in the grid and the other 2 thematics interchange. It seems to give me a grid full of real words and also seems to match the preamble.