Does this mount to the factory bed rail or does it need drilling?

These clamps attach at factory mounting points — no drilling required. You're working with existing bed hardware, so installation is straightforward and reversible.

Can I leave this folded down while driving, or does it need to be fully closed?

Tri-fold tonneau covers are designed to fold completely against the cab for full bed access. When folded, they sit low enough not to obstruct rear visibility, but confirm your local visibility laws before highway use.

What's the warranty on the fabric panels if they tear or wear?

Specific warranty details aren't included in the product listing — contact ModSupply directly with your order number for coverage terms and claim procedures.