Well if the charger can charge the 5S one at 5.8A go for it I was just saying depending on the wattage of the charger. If the charger is rated for 100Watts you are fine at 5.8Amps(I would do 5A to not stress the charger though) as it would require 99Watts to charge it. If its a 60 Watt charger 3.3amps would be about the maximum it could handle charging a 5S battery.

Slow charging lipos I don't see the point if it saves anything it would probably be negligible. I mean like 10 seconds more run time after 70 cycles you won't notice if its true or not
As long as you don't charge them over their ratings they should be fine which is why I said when in doubt go with 1C charge rate.