Spent some time on test tonight and it's implemented worse than I thought.
FTE encounters are actually pretty fragile and the mobs seemingly pop out to FFA randomly. This is because social/yell for help aggro only locks them to the encounter as if you feigned, so like 15 seconds. When mobs drop from the FTE encounter lock, they go FFA and do not lock again unless they are fully blurred for 15 seconds.
It was also very easy to peel grey mobs onto healers and other people and give them problems they had no response to. According to the devs on discord, the idea is that if a mob attacks a PC that is outside of their encounter, the encounter will break and the mob will become FFA.
This is very easy to exploit (especially early in an encounter) and makes it almost useless for any meaningful KS prevention. You can heal an encounter off and then DPS race the mob as usually and get credit. Low health aggro also seemed to work, so worst case you could have someone in your group nuke themselves low and break the encounter that way. Additionally, we found that a bunch of solo players sitting around a spawn could fully DPS race a mob before it even locked to any encounter. Two people in a group seemed to tag it over the solo players, making it even more important to run with a box or a group if you are in a camp. This doesn't even include things like dragon AEs 'hitting' non-encounter people and having it break the encounter to a straight DPS race.
Ultimately, it doesn't solve KSing, AE PLing nor training and in fact, leads to all kinds of additional degenerate behavior. As it was on test, the guards wouldn't assist people who were dying (even after a /yell) and individuals with hard aggro had to specifically target each mob to /yell it off. Meanwhile, pull an entire zone with social aggro and flop and every mob in the train becomes FFA and obeys traditional dps/xp rules.