Question: Do the runes, Font of Life and Revitalize stack?

Yes, but if you're playing an engage tank support I'd say Overgrowth is better, Revitalise is more for Enchanter supports that will get the most from it

As a tank your hp bar is often a resource that you want to spend in a fight, the more hp the better, overgrowth is designed to be viable for tank supports

I'd even take unflinching over revitalise

Why wouldn't it?