Does this bolt directly to my stock catalytic converter?

Yes. This connects at the cat flange, so your existing converter stays in place. You're only replacing the section behind it.

What about clearance — will this hang too low or hit the rear bumper?

Mounting geometry and ground clearance are not specified in the product data. Test-fit after installation or reach out to confirm fitment on your specific vehicle before finalizing.

Can I use this system with a catted downpipe later, or does it lock me in?

Catback systems are designed to pair with factory or aftermarket downpipes at the flange. This system will work with either, so you're not locked in.