No point using tower shield because you are not proficient with it. I can't give you advice at the moment because I have no idea what class your party member took. I would say. 19 AC is not bad for a cleric. If it's a hallway and they can't get pass you if you stand still. Have your partner stand back if he has less AC than you, then total defence to create a bottle neck effect. If not, stand back and let him do his thing. =)
You can't fight defensively and heal someone in the same round. (At least, not in my game. You could try to argue that you're casting cure light wounds like a touch attack spell on an ally, and that you're making that attack defensively.)
Assuming you don't have Tower Shield proficiency, the accuracy penalties (-12 to hit) mean there's no real point to you attacking. If you're not attacking or casting, then the Total Defense action is better than fighting defensively.
The thing I've learnt about being a support, throw out 2 buff spells and get into the fighting after that. It's very rare to find fights that last long enough to justify more than 2 buffs.
If you can prebuff that makes a huge difference.
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A CRUSADER archtype Cleric can wield a tower shield no problems and if you use smart positioning you could cast defensively and never take attacks of opportunity.
With a tower + scale you'll be around around 19 AC before feats and other bonuses, it's nearly impossible for any level 1 creature to hit you.
Prebuff means to anticipate when there's likely to be a fight and cast the spell before it starts. This depends on you not being ambushed and is hard at level 1 when you have limited spells and short durations.