Assuming I have not misunderstood the instruction, I have found the 4-word thematic, and notated the single word thematic, so the two clues that I have not answered yet - 12ac and 16ac - may remain unanswered...as their unches are irrelevant.
Not sure how I feel about a crossword that doesn't require all clues to be solved. The answering of 12 and 16 do confirm the information due to their missing letters, but the crossword can be completed without requiring these clues (and others) to be solved..