I find urethane to be very forgiving, btw.
Urethane is very sensitive to any moisture that may be in the model, the board it sets on, even wood tools etc., silicone is a much better more forgiving mold compound that doesn't have that problem, in fact the rubber I use can be molded over moist or wet clay with no problem at all.
Urethanes that pick up any moisture can stop curing and wow what a mess!
On the mold making and damage, making a mold of a delicate, old original, or an original carries some risks, the rubber could react and fail to cure, (common) the rubber could be defective as a batch and fail to cure (rare) the rubber can stick like glue to the original despite use of release (had that happen with a wood carving, I had to burn scrape and melt the rubber off it)
The mold maker can **** up, it happens.
If spraying on a sealer or risk of damage is unacceptable to you, then don't have a mold made.
The only rubber I know of that Ive used twice on delicate antiques, both were painted wood, is Body Double by Smooth-on, it will not stick to anything, no sealer or relase was used, and is a simple 1:1 mix, ready to pull off and use in less than one hour. It is VERY expensive and for large pieces the cost would be prohibitive.
It was around $100 plus shipping for 2 trial kits, about 4 pounds worth, the 2 trial kits I ordered was just enough to make a mold of a flat plaque about 10"x 10" or so.