@pinkeree
Need to note that this is just my interpretation. I feel Phos wanted to just destroy all the gems because in Phos' eyes, Gem now were the one who actually hinder her objective to save lunarian and in which will save all the gems (restoring gems using Lunarian technology). The problem was simple actually, it's all due to misunderstanding (there's no real conversation between phos and earth Gem so far), and Phos sneaking around made it worst.
By being free, I think it's about being free from the shackles of duty to save everyone. Phos was strongly obligated to save antarc and other gems who sacrifice themselves for Phos (or at least on her mind), and she has no intention to back down and will do it whatever the cost. Turned out the cost was too great (being the enemy of the earth gems and being obliterated into pieces). If I were Phos... well... I think I want to be free too.
About "keeping that from me", I think it's more about "keeping the deal to leave earth on Phos' hand". The prince was asking whether Phos still wanted to try to make Adamie pray. Phos' answer was basically "yes... I will try, but I need to destroy the gems first (because they are now an obstacle)".
The dog smashed her because she show hostility to Cairngorm... the dog was the gift from the Prince to Cairngorm, to protect her, I assume... so yeah, shit happened after that