The responses you received for your questions numbered 1 and 3 are not as accurate as they seem to be.
1. You can tow with a short bed without a slider. You just will not have the ability to make turns much over 70 some odd degrees without hitting the cab.
3. Here is the reference from Lippert Components: https://lci1.com/goose-box
I believe the model in question comes with the Lippert 1621 pin box.
FR does not warranty the frame and has no say in it. LCI does. The notion that you would totally void your warranty is rejected (although they didn't exactly say that). If you are interested in the truth, I suggest you contact LCI directly.
FYI, I was the recipient of false info from a FR Warranty/Service Mgr not long ago. A visit to the factory straightened that out. The good news is that person is no longer a FR employee and my issue was satisfactorily resolved.