Hm, I don't think anyone is claiming to be too expert to be wrong here, I at least err more than enough. If I come blunt it's just that English is not my native language (the usual disclaimer), so apologies for that.
Anyway, I think you've noticed too that about the only problem with using your pretender as a lone combatant is her low protection - no body slot nor earth magic. Should you manage to up her protection somehow (and get some regeneration) she's a melee monster, with fire shield, breath of winter and personal quickness. Barring that, site search, research, forging and summons then, with an assisting role in battles in a pinch. Note that if you go Evocation tree, she gets all the acid spells which are AP and armour-destructing. Very yummy.
As I said, Agartha gets nice battlefield spells early with a variation necessary for changing opponents. Human troops get stuck with Earth Meld (Alt2), high protection can be negated with Rust Mist (Evo2 if you get it to work) and Magma Bolts kill even giants (Evo3). I'd hope those would get you through early-game rush should you start near some stupid bless-nation. Note that aforementioned things rely on you bringing Golem Crafters into fray. Earth Power and Stoneskin may help with their survivability. Also, don't forget the sitesearching spells from Thau2 and Conj2.
I'm not an expert with construction magic, but I'd think if you can get a good gem income Const4 could be handy with your Ancient Lords (Frost Brand, Charcoal Shield, Elemental armor give 100% prot against two elements and fire and cold elemental damage irrespective of attack skill, just add reinvigoration). ...damn, I really need to switch to Construction in my MP game...
I think much depends on where you're heading and who is against you. Earth Meld is bound to be totally useless against Jotuns but a lifesaver against a Vanir rush. I can't tell what's the truth, just my version of it.
And what comes to holy sites, and sitesearching in general, I wouldn't stress too much of finding everything. Just use your pretender to search initially, then forge a couple of fire/water boosters to your Crafters and have them cast sitesearching spells on all your provinces.