12a Attracts is the definition, and the change described by the second instruction has affected the beginning.
14a The last two letters are CA, which should have a letter in between, in accordance with the second instruction.
18a Your last three letters are correct -- to become AC?T
20a Alternative letters of edelweiss indeed, but lose the S, and the N is right, and follow second instruction at the start.
Having said which, I have the two thematic entries, but am not clear how to apply the first instruction to them.