This is getting exasperating! Yesterday, I charged the battery with
my 10A charger until the battery showed 12.3V. Then I left it
overnight with my trickle charger connected. This AM the battery read
12.4V. I cut the lawn (2 hours) and then checked the battery. It
read `14.1V! This tells me that the mower alternator (?) is working.
I left the mower off for two hours (with the battery cutoff switch
OFF), and the battery voltage was 13.3V!

The above voltage change is normal

Right now, it is some 4 hours later - the battery reads 12.3V and the
mower starts, but with its usual reluctance.

It sounds to me like the engine turns over tougher than it should, or
the starter is inefficient for some reason.