I appreciate your tailors' attitude, but I think we might approach this differently with a happy enough result I read somewhere the following advice:

If you can't take the back off, you don't really own it.

Meaning, it is yours, and if you choose to risk ruining it, that is one of the rights of ownership.

Please understand, a smart tailor might well run from velvet, but you have what is in effect, a roadkill jacket. It is already dead. You would not ever wear it in its present condition. If someone can make it wearable, you have harvested / salvaged something that would have gone to waste. With no cost / no pride of ancestry, you can also attribute a certain devil-may-care quality to this project. Granted, it would be nice to make a beautiful jacket, but making one that is merely pretty good is not such a bad thing...

NOW, considering the treatment you got from your tried and true, it might be worthwhile to assess how that influences your design choices. On the one hand, you might opt for the simplest amount of work, that being the least number of opportunities for mischief with the fabric. OR, you might bear in mind that the greater the amount of ornamentation you put on it, the less people will notice any flaws. Think of the difference between a piece of beautiful cut crystal and a beautiful piece of absolutely plain crystal. The one is valued FOR its ornamentation and the other for its sparseness. And its flawlessness. So, fewer changes or more raised seams, buttons, and contrasting facings?

One final factor, though: If this project seems doomed from the start, you might not want to plow too much cash into it.

I'd look for an immigrant tailor who will do a lot of handwork and be sure to warn them about steaming / pressing. Best of luck.