Struggled with this away from home and PC. Now back home, still can't suss out 20a: "offer to divide charge for constituent of acetylene". Got C-R-I-E. And I'd like to know why 35a is LODGER - "one isn't necessarily accommodated in the gatehouse".
Thanks, chrise. And, after some thought, I now get the offer (bid) in the middle of charge (care). As for the scientific correctness - or otherwise - of the clue, that's way beyond me!!