During waiting on battle and searching for a group, you can turn on a switch telling system that you can be used as a reinforcement. That means, that when someone lost connection or just leave the battle before specific time (maybe you can set it for yourselves), you can be chosen to replace him and you get a confirmation dialog for that. In order to prevent the possibility to enter an already lost battle you will reinforce only battles where the time left compared to difference between scores is reasonable. This way the leavers will not ruin the battle as much as it is now when they leave at the begining. After the point where reinforcements will not come or nobody is replacing the leaver, he can be replaced by a henchman.
After all this is war and reinforcements are vital part of battles
.I think that there are many players who just enjoy grouping and playing when they don't have friends online so they are a perfect match for this.
Possible bot protection:
For stopping abusement by bots there can be a periodicaly generated random noisy character code you must write down like on webpage confirmation forms. After x-th battle the option is automaticaly disabled so you must reenable it and write the new generated code.
. But the second indicator (as you mentioned) can be that the team isn't loosing too much, so it will be considered as the battle is already lost and the reinforcements are a waste of time 
