I like 'ARCH', elliemay. Unfortunately that makes 26A 'exhaust gaffer' begin with 'H'. I thought of 'VIA' for 24D 'By', but if I'm correct with 'RELIEVED', it begins with 'E' not 'V'.
Not such a silly idea, as CREM is a word, albeit an abbreviation for crematorium. However, while it is indeed MERC backwards, I can't see how it fits with the '8' in the clue - the answer to 8A, by the way, is 'FAIR'.
Or it could be 'TRAM' which is 'MART' backwards. That would confirm 'MANIFOLD' for 26A (though I don't see how it means 'gaffer'), 'PLAN' for 22D leaving 24D 'By' (3) E - F. Hmm.