I could have almost written this myself, Pantheropad. After a struggle, I've got the spare letters, all squares filled, theme, the two unclued etc etc.
I might also have got two of the first sort of new entries, but now totally stuck, despite the well meant hints above.
Do the 10 substitute letters still mean that any crossing words are still words after the changes?
Any new hints would be gratefully received. I'm on the point of abandoning the whole project