In 13d, the definition is 'Bill' (of exchange) and the redundant letter is the 'a' in reactor. The second word is given by the wordplay, 'is enthusiastic about re(a)ctor'. I hope this helps.
My difficulty is with the nature of the interleaving. This doesn't seem to be by alternate letters, I can see some sequences of letters that yield words, but, as a whole, what I have lacks coherence.