Eh, I think you're underestimating Val's defensive potential.
Shield spell is +4 AC, period. Tower Shield can go up to +9 AC (at +5) +2 Greater Shield Focus, +1/2 Shield Wall teamwork. So that's up to 8/9 AC more. Also her class removes the attack penalty at level 5.
True, for her to tank anything remotely dangerous above Normal, you have to buff the shit out of her (Barkskin, Shield of Faith, Blur, Magic Vestment, maybe Cat's Grace, Protection from Evil) and give her all best defensive stuff (including Dex increasing items, as her class also increases Dex bonus allowance for armor and Tower Shields).
You could go crazy, keep her weapon hand empty and go for the Crane Style defensive feats. That might be going too far though, I think. Particularly that there is supposedly a +3 dodge AC dagger for hardcore defense mode somewhere out there.
So IMO just mix her Tower Shield Specialist with Stalwart Defender, when available. Later on you can dip Vivi for a little extra Natural Armor, sneak attack and Dex or Str.
Offense wise, I mainly use her for Dazzling Display aoe shaken.
Of course a custom made, optimized defender build would be considerably better. And her Will save is a pain.
Edit: Jaethal's immunities are great, but I don't think she can hold a candle to Val AC-wise.
Look, I finished chapter one with her tanking, and she performed poorly. It's great that she's finally becoming good at it at a high level... but who's playing the game until then? I didn't have someone who could cast Barkskin, Blur etc in chapter one. You? All those builds that take 10 or even more levels before they stop being worthless are... crap. For me it's more about being able to tank (or kill stuff quick) when it counts at the begginning, looking at higher levels I'm certain jaethal can be just as good - better actually due to better stats and irrelevant will saves - after all, nothing stops you to do all that stuff with Jaethal as well.
In addition to NOT being able to survive, Val also contributed hardly anything to the offense. Now in the new run with her fighter 1/sorc 2 she has the same AC should would have with tower shield (at that level) but actually kills things. She also has Enlarge. And when she reaches the level where she would have had equipment like that +5 tower shield you mentioned she will have sorcerer casting granting survivabilty and far more utility / offensive power than she would have as a fighter. So, no I don't underestimate her potential to finally be able to tank at level X, I merely don't see her performing at all in chapter 1 and 2. If you know how to build her so she can perform that role on hard in ch1/2 then spill the beans.
My question regarding her wasn't to tank or not to tank (answer is no imo) but whether to go for 4 dragon disciple to get that +4str +2ac or to go for EK immediately. Sometimes when sufficient defense isn't achievable (early enough), overwhelming offense is the only feasible way.
Jaethal I already gave back to back so she always has +2ac when flanked, the question is stay inquisitor or go something else. Go fighter for more feats to get improved unarmed fighting and crane style? Go cleric? How to increase her tankiness as quickly as possible? If you go s&b ranger will l6 shield master counter the penalty from tower shields? Ranger would let her cast barkskin for instance and lead blades.