Sounds like your CSR was mistaken or the web site is wrong. We do know that Denver does have HD locals for channels 2, 4, 7, 9 and 31. You might just want to call them back and make sure which is which.
For OTA, the $50 gets you the AM21 over the air module. Is it worth it, if you MUST have your PBS in HD then yes since DirecTV doesn't carry it or KTVD. It's also nice to have a backup in case the satellite signal goes out due to rain or snow storms.
As for what HD DVR you get, it's probably either the HR21 or HR22, all depends on what gets loaded on the van in the morning. You can't control what you'll get, even if a CSR says you can. Both of these boxes don't have OTA support unless you add the AM21. The only difference between the two is the HR22 has a 500GB harddrive vs. a 350GB which give you more recording time.