Williamseal - I think you’re on the right lines, I don’t think the end game quite works, unless I’m missing something. I think the end game is a 10 letter word in English, with the 5th and 6th symbols each representing 2 letters.
15a is clued as having the 2 letters, 14a isn’t. The 2 letter symbol at 14a looks like an E, so 14a works only if you read the symbol as English. 15a only works if you read the symbol the thematic way.
Would be very happy for someone to point out where I’ve gone wrong and that this does hang together, as I thought the puzzle overall was very clever and very well done.