[ ] FIRST LINE: Caldera
[ ] SECOND LINE: Withdraw
[ ] THIRD LINE: Eastern Valley
Seems the best way to get at things in my opinion.
Caldera allows us to murder the most Orks possible and possibly shatter their leadership, as well as keeping whatever siege engines and heavier troops they have, slower than the line troops, be in range of the Eye. Murders them something fierce and maybe (fingers crossed) breaks their morale and disorganizes them.
Withdraw allows us to conserve forces the most as well as bleeding them. What seems to have been overlooked in most of posts I read is that the Ork groups attacking with the Caldera First Line plan, and thus after taking an Eye glare, will be finite and separated from the rest of the main force and will basically remain so because the Eye will take care of any possible reinforcement from the West Gate. What remains to do then would be to attrition them into oblivion to win. Either option works for that, but withdraw would allow a better force conservation.
And then Eastern Valley, where the attritioned and likely disorganized Orks smash against all our best advantages: the gyros, the Eye and the fortifications set up previously to repel the Trolls. Here we could also see the consequences of either previous option: If Hold, a severely depleted Ork force will clash against a severely depleted defense force. If Withdraw, a likely less depleted Ork force would clash against a fresh and at near full strength defense force. Again, withdraw seems to be the better option since it would let us have the most forces in the best conditions for us.
Would be the best path for force conservation, which would allow a possible sally against the remnants in K8P later on and maybe against the ones outside thanks to low casualties and fresh troops.
Another point to take into consideration is that we are unlikely to get reinforcements in a relevant time frame, so what we have is all that we'll likely get. Attrition against a numerically superior foe is the real killer here and should be avoided at all costs IMHO.