Will these hold up to exhaust heat without melting or loosening?

Stainless steel handles direct exhaust temperatures — no thermal breakdown. Tighten by hand until snug; don't over-cinch the wrap or you'll deform it.

Do I need these for both the wrap layer and any outer sleeve?

One tie per wrap bundle is usually enough, but if you're running a protective outer layer too, plan for a second tie higher up the bundle to keep everything locked in place.

Can I reuse these if I need to unwrap and re-secure later?

No — once tightened, stainless zip ties lock permanently. Buy extras if you plan to adjust your wrap later on.