Further suggestions:

With respect to plastering:

That as soon as the insulation has passed inspection, and not before,
we schedule at the house, all available plastering sub-contractors, one at a
time for bids on a mutually convenient date. We shoud be prepared to sign a 
contract that day upon receipt of all bids.

Non-binding estimates for a future contract seem to me ineffectual;
But as always, I'm not at all insistent. The decision is yours,