Yeah PvE god roll would be the same for both. They're basically the same weapon except Avalanche has worse stats across the board and is time-gated. I'd still keep it even after you farm a FF + Rampage Hammerhead because a Solar LMG might come in handy.
That sounds like it could be useful but personally I swap to Luna, drop rift, and swap back to Geomags when Primevals spawn (luckily I am on PC). You could use Rally Barricade or Marksman dodge as well. But hey, you aren't limited in how many times you can farm Forges for any particular roll which is the great thing. I know I'll be going for something novel like Shield Disorient + Genesis once I get my own god roll ready to go.