Anyone in Denver CO who can rent me a Bionix downtube battery?

Ship your battery via Ground UPS or FedEx to your destination. Bit of a hassle, but a work around.
Yes it can, but you pay fees to do it. Our shop occasionally sends the batteries off for repair or replacement and ground shipping is the only option but you have to have someone who is certified to do hazardous shipping prepare the box.
Yes but since the places that will do it on this end are a 2 hour drive away it's an additional burden--not to mention it needs to be done on both ends of a trip.
That doesn't mean it can't be done, you're just saying it's not worth your time. Call Tom at Small Planet E Bikes here in Denver.