Can low coolant level in radiator cause valve gaskets to fail and cause white smoke to come from rear exhaust

If the coolant is low enough to allow the engine to overheat, yes. And yes, white smoke indicates coolant entering the combustion chamber.

Sounds more like head gasket. The coolant is low because it's getting pushed out of your engine and into the exhaust. Try Permatex Head gasket repair in a bottle. I NEVER thought this stuff could work, but I saw it with my own eyes. It's like $30 at autozone. If you have has the head gasket issue for a long time you could have done more extensive damage, more than this magical bottle can repair, but it's worth a shot.