prospero - look at the letters you've removed and, for many of them, take into account the preceding letter too (that remains in the word you entered into the grid). This should unlock the Greek for you..... well, it did for me anyway :-)
I can't see how it works for 5. I haven't got 18 either. 28 is just the first four letters of the answer (Eastern puzzle) I think. Is 19ac simply dons or have I missed something?
I think it's not a theme - just a device where you take into account at least one extra letter which is an abbreviation of itself plus what you take out. In 5d what you take out are letters 5-8, not the alternative. 19a is an American term for academics - but I need an a in it to make the device work ...