I plan to dye a lined, nylon parka (i.e, bulky). Could I use a large galvanized tub as a dye vat?

A power grinder with a thick grinding wheel is more suited to this purpose. They retail for about $30 CAD and are a welder's best friend.

Hot-dip galvanization gives a thicker and uniform anti-corrosive surface layer after molten zinc immersion.

Get a great haircut from someone experienced and skilled in cutting curly hair. There's not really any other way around it.

You must use fabric dye to make your jeans black. If you use hair dye, the color will quickly and easily fade.

Yes, it will work well for single-color tie-dye. An experienced dyer could achieve more colors, but since you must soak the nylon in the liquid rather than squirt the dye...

Try it out with the normal method detailed above, but just do it on 1 piece of yarn so you can see if it has the intended effect without potentially...

Yes. The higher the wool content, the warmer and drier they will be, but 85% is great.

You can use a tarp or any plastic sheeting, just be sure the material is strong and won't tear.

You can put them in a spare closet, or you can put them in the back of your regular closet during the warmer months so they're not in the way....

Wear lightweight layers instead of heavy ones. You'll stay warm even in cold weather, because the space between the layers will help add warmth.