Yes, it is. P750GM and P850GM can "explode", but only in case of overload (only the two "primary switching FET" can). Up to 850W you won't have any problems, it's more than enough for a good graphics card and for a normal PC. For OC buy another one.

No, recommend wattage for a system containing a 3070 is a 750 watt psu, the max draw of one 3070 is 320 watts so not enough overhead for 2 x 3070's. I would recommend a 1000 watt psu as a minimum.

You probably have a limit of 2 on your motherboard so that's no problem

https://www.gigabyte.com/Power-Supply/P850GM#kf