1) Depends upon the charger, some charge correctly some don't.
2) A charger with maintenance mode or a maintenance only charger. Or fully charge then disconnect the batteries, I have a switch that completely disconnects the batteries, mfg switches usually leave some loads. Fully charge every month.
3) Google search for LNB's IF that is actually the problem. Do you have DTV or Dish?
4) Inverters convert DC to 120V AC, nothing to do with battery charging. If you mean charger/inverter combo then answered above.
Once the plates are exposed then that portion of the plates sulfates and you lose capacity which cannot be restored. Batteries could be weak or shot and voltage alone doesn't identify a weak battery. If weak but adequate for you then OK. BUT have them load tested so that you know. I'd go to a good battery shop and auto stores are not a good battery shop.
Adding acid changes the SP Specific Gravity and the battery will under preform. NEVER add acid.
I'll venture that there is a 99.9% change you need new batteries.
