BLM theorycrafting aside...

The THM in your party wasn't doing anything wrong, he was just a jerk. You insisting that it was impossible for you to gain threat over more than 1 target at a time would easily incite the rage of many DPS players who aren't patient or kind. These problems will melt away as you get all of your skills and everyone becomes a little better at their class through leveling up, but tanking pre-40 as a GLD/PLD can be a little sketchy if you are not experienced and your DPS is experienced.

General skill advice would be to body pull with Flash x3 or so, RB some of your mana back, check your enmity and apply a RoH combo as needed, flash a few more times, etc. If you focus on managing enmity at lower levels and pushing yourself to hold enmity over any a-hole dps that comes your way, you will be an excellent tank when you get to level 50.

I won't pat you on the back and say you were completely right here, because if I was a THM in a dungeon and my GLD said I wasn't allowed to use one of my most powerful DPS spells on 3+ targets because it was impossible for them to hold enmity my reaction would probably not be, "Oh, cool." It would likely be to explain to you that if you drain all your mana holding enmity over me each pull its fine because if you front-load your enmity generation with flash you can spend time RBing it back up before the next pull starts.

Iunno.