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I have a full grid but not sure how these 2 clues work if anyone can enlighten me please:
In each clue the wordplay has an extra letter

16a Instrument's gone missing, absent for end of largo (3) A?L answer must be AWL?? the extra letter needs to be "A"
24d Personal trainers could have been somehow politer with these thorny ones - answer is NARRASES - extra letter here is "N" (narras is a thorny shrub)

I have the 8 word message spelled out but unsure of where to go next.

16 AWOL = absent, substitute A for O = AWL + extra A

24 anagram of narasses + politer +N = personal trainers

EV likes to use the diagonals.

sorry, AWOL = gone missing, absent = A

Thanks kilgore trout - i thought awol must be involved somewhere.
And i've done the changing now too1

I have had a full grid since Sunday and also the instruction but for the life of me I cannot see what needs to be changed despite looking at every diagonal from every direction.. Could some kind soul please point me in the right direction before I tear any more of my hair out. Are the letters in one complete sequence or are they split?

It's one complete sequence down a long diagonal.

If you have the instruction you will know that letters
must be changed to make the creation the creator.
Look at the diagonal from top left to bottom right and
you should identify a name connected ( in poor taste
I felt) with the title.
If you change 10 of the letters you should recognise
the creator.
I should add that after the changes you are left with real words.

May thanks Cloverjo and Tony for your help. It is much appreciated. I made the fatal error of looking for a sequence of only ten consecutive letters which needed changing and did not "mind the gap".