Usingf the remote, I can adjust the volume in the headphones from eight feet away, no problem. The remote has no backlight, unfortunately.
Concerning front vs rear USB inputs, first note that the supported list of Apple products in the manual includes various iPod and iPhone manuals, but there is no mention of an iPad. I don't own an iPad, but my iPhone 8+ works fine through the front USB port. The manual says nothing about plugging Apple devices into the rear port, and I've never tried it. Obviously you'll need a Lightning-to-USB/B cable, which seems to run $35-40 US; I don't have one. I know that files on a USB flash drive will not play from the front port unless they are 16/44.1 (CD) quality or below.
I have a 4400h, and I can tell you that CD-quality and hi-res files sent to the DAC1 from an Oppo player via optical or coax, and then sent analog to my Denon 4400h, sound noticeably better than when the Denon's DAC does the conversion--better imaging and wider sounstage.