you can use any battery to start your lawn tractor. The thing is bigger battey can retain for no. of starts. But the charging system provided in lawn tractor is not sufficient to chage bigger battery. Hence you have to charge it by bench charge of fitting in to your car.

Only If the voltages are the same. Otherwise you risk damage to weaker battery.

Yes, that will work if they are both 12 volt batteries.

Sure. I'd try it first with the car engine off. Just hook up the cables and see if the reserve is enough.

as long as it is a 12 volt system also,yes.

No.