There have been times when enemy players have appeared "friendly" in which case "TargetEnemyUnit" would not end up selecting that player. As already stated, you can infact target the stone transformation players so this is in fact NOT the problem you are having. There is some other issue at hand, prevent the TargetEnemyUnit from selecting the enemy players in question.
And yes also as already stated, said transformation potion is nearly impossible to click on but this does not prevent ANY targetting.
Nice to know the hp effect is intended, I was worried about stocking up on them or not o.o
Final note, I have decided not to use this xform pot in sw unless I see enemy players using it as well. I can't speak for my team mates, but I feel it is unfair for one team to use it but not the other -- if both use it, then all is well, if neither use it all is also well

It makes for a really fun sw due to not knowing if the rock on the side of the road is a friend or an enemy lol