I don't think any of the retaliation enchants actually 'stack' as such. So in your example, the multiple enchants might result in multiple retaliations - up to 5 if you're lucky, but each one would only do 35 damage.
Although, come to think of it, I have "static charge" on most of my spare enchantment slots, and I've never seen more than one 'bolt' fly out when I'm hit. But I've never paid that much attention - these enchants always do negligible damage compared to what I can do with my own spells. Maybe someone else can confirm for sure.
Good luck getting your tower
