So, if you mount your Shaman on an EOTG, it automatically gains a level, right? Plus the level upgrade is a level 3 Skink Shaman, right? Throw a Plaque of Tepok on him, and you've essentially got a lvl 4 Shaman, correct? Is this a wise investment in points? With a Slann? With a Carnoblood?
Not quite, giving the Plaque of Tepok to the Priest makes him level 3 on the dot. The Engine does not grant an extra spell, only extra dice as though the wizard was another level higher.
He'd be a Lvl.2 Shaman with 3 power dice, 2 d.dice, and knows 3 spells. I don't know how you got the Lvl.4 in there, but it would be awesome.
Its not that bad, especially when you consider all the heaven's spells are very situational. I also equip mine with a 2+ ward save against the first wound he takes. Saves him from bolt throwers and is miscast protection.