If acetate taffeta, it looses it strentgh and luster when wet, usually dry cleaning recommended.
Some can be washed cold in cold water, hang to dry.
If silk taffeta it can be hand washed in cold water.

Cut a small piece, wash it, see what happens, some taffetas are a blended with polyester.

What would be the worst that could happen, say IF it got put in the washer, cold water, gentle cyle, then air/fluff dry? no heat?

It's like 6 yards of fabric, a big challenge to hand wash. I have no idea what I'll do with it, maybe I'll just try to sell it in my antique booth? or might use it to cover some little chairs for a princess motif vanity set I've got in mind with some new CL hand made wood vanities, cute as a button!

Can you find a "bulk" or DIY dry cleaner where you pay by the pound?

You could do a burn test on a strip of it to determine what the content is, then figure out which cleaning method to use. I added a link to one of several sites that tell how to do it.
Otherwise, I think the method you were thinking...cold water, gentle cycle,air fluff...would be the way I would go. You'd know the results before making something, and if the fabric lost its hand, etc, you could always use it for lining or facing, etc. I don't think you would have much to lose since it would be hard to sell due to the odor, anyway.

Here is a link that might be useful: burn test

Hello, I have just bought an antique fabric knitting needle box. The fabric is ok but smells and has a few stains. How do I clean it?
Thanks for any tips

Is it dirty or stained? Or does it just smell musty? If it's just musty, hang it outside to air out in the sun. A day on the line will do wonders for it. That's what grandma did when she brought out bedding that had been stored for a year. No line? Drape over a fence or set up chairs to drape it over. Necessity is the mother of invention. Put on your thinking cap. I'm sure you'll think of a way. If there are just a few stains you can probably spot treat those.