Pretty sure that 14.3 VDC wouldn't hurt anything in a 12 VDC system. Keep in mind that the 50 A setting just means the charger is capable of delivering up to 50 A, Doesn't mean that 50 A will be delivered the entire time it's connected...
In all honesty though, the 10 A setting wight be sufficient, I don't think that the starter on our thumper's would draw 10 A...but then again, I've never thrown an ammeter on my battery wires when cranking...lol
Sounds like the battery might be shot if it's dropping that much with minimal rebound.