Yes it's TK and yes he doesn't buff. .. but his banner raise dead models within 24", and the ability to bring back for free a necropolis knight is not something to laugh at.
Also, if you have oxyotl, couldn't you put down Oxyotl with a 10 chameleon skink unit around him in terrain (3+ defense), and cast your lord kroak's mystic shield through him on your 10 chameleon skinks (2+ defense), and maybe use him to summon a skink priest through your oxyotl for rerolls? That way you can say put down a remote second battle front with 10 2+ defense units with rerolls on saves.
There are two problems with it. Lord Kroak doesn't have the Arcane Vassal ability, you need to use a Slann. The mystic shield and the summoning go in the hero phase, chama skinks and Oxyotl appear in the move phase, so to obtain that 2+ save re-rollable, you need to have them appear with "just" a 3+ save, and then wait the next turn.
Sadly not. The arcane vassal is an ability that you find only in the battlescroll of the Slann Starmaster and there's not in Kroak's battlescroll. ( unless there is an errata i'm not aware of)
Ah, that's a pity. Also do you think when they update the Seraphon Allegiance the Slann will get some buffs? They are too far orientated towards summoning, and with point costs that has been ridiculously nerfed.