Two six volt batteries in series makes 12 volts so switch the charger to the 12 volt position.
yes
Yes, if the two 6 volt batteries were connected together in series.
Two 6 volt batteries in series, yes. In parallel, no.
No!
Depends on the amperage of the batteries you are using. Both setups will give you 36 volts, but you will normally have a longer run time with six 6 volt batteries in series than three 12 volt batteries. That is why golf carts use 6 volt instead of 12 volt batteries.
Check interstate battery, if not check FVP a subdivision of Delco batteries.
Paralleling four sets of 6 volt batteries will still give you 6 volts. Two 6 volt batteries in series will give you 12 volts. Parallel these series sets will give you longer endurance at 12 volts. There is no way you will ever get 48 volts out of four 6 volt batteries.
You cannot use a 12 volt charger to charge 2.6 volt batteries. The 2.6 volt batteries will become hot, very quickly, and explode.
There are 6 volt car batteries, but you are only likely to encounter them in really old cars, like late 50s early 60s.
Yes
A group of ten (10) batteries composed of 6 Volt and 12 Volt types will have six (6) each, 6 Volt and four (4) each 12 Volt batteries. The contributing voltages will be 6x6 and 12x4 for a total of 84 Volts.