From what I understand guilds are account locked meaning I could have a Dunmer and a Khajit in the same guild.

What is to stop half of the 300 guild members from logging on as the Ebonheart Pact and the other half logging on as the Dominion and farming alliance points ?

Is there a mechanic to stop this from happening ?

I believe they said that you would not be in the same Campaign as the other factions even if you are in a guild

this is confirmed +1