Sorry this isn't turning out as you hoped. I read some misunderstanding between buyer and seller that will be hard to overcome, especially since the timbers have been in your care for 12 months. Usually you get a few days to bring up disputes. Anyone selling standing dead should quiz the buyer hard because it always has lots more 'character' to deal with. For that reason I find I need to order more 'extra' sticks to be able to discard the odd wanker (usually 10% extra). Owner must be willing to accept some bug evidence and existing big checks - in exchange for a more stable wood.

Sounds like your 'dead' may have been caused by bugs rather than fire or other cause. Borer evidence is common and expected in SD timber, particularly fir, and yours might show more than 'usual'. No mill would agree to 'no bug evidence' unless you paid a ton for it and even then maybe not possible. Bugs should be gone if stick is well seasoned and quite dry. If you got lots of sapwood the bugs will show more and wood not as durable. I find more problems with ant nests started in big checks.

The high MC indicates a recently killed tree. Were they checked a lot off the truck? You should have seen 1/2" or better checks commonly (and blackened by dirt and sun) if the sticks were indeed dead and well seasoned on the stump. Typical SD fir or larch will be quite dry because it has been standing in the wind for years - stability one of the reasons I like it. If recently dead, all the fun movement happens when MC drops from 25 ish into the teens. Some movement would be expected since your MD weather quite a shock to the sticks.

Welcome to the wacky wood world.