Alez0, it may be a miscommunication but it sounds like you are misunderstanding something. The Rearguards are not visible on the screen and cannot be hit. They are invulnerable and therefore never "downed". Even if the Vanguard is downed, and therefore all players on the screen are down at zero hp, the Rearguard is not dead.
However, the bigger issue here is that the battle system is, in fact, not perfect yet. But it is a brand new game with a complex new system, so give them some time now that they've seen how it is being played, and see if they make positive changes.
I know for a fact that the GM's are looking into any possible tweaks to make it even more fun and competitive, and less vulnerable to any underhanded strategies. A vast number of possible changes are being considered, so just because it isn't happening right now doesn't mean it never will! They just have to be careful with this lovely game and carefully consider any changes before they commit to them. If they decide something will be a great addition to the game though, to make the battle system more dynamic and secure vs. cheating, then I am sure they will do it!
We just ask for your patience. Anyway, I hope you're enjoying the game overall!