My point was (as I understand the situation) the Captain does NOT have to accept any particular M.E.L., however his Company will expect a good reason why.
With multiple M.E.L.s, that is usually taken into account anyway.
An example would be on a twin, say one generator/alternator U/S. The M.E.L. would normally state subject to the A.P.U being serviceable, and other things like it would have to be a non ETOPS.