Hi,
I've seen DP write to USB-connected HP DATs, so that isn't entirely impossible (it was DP6.2x on W2008R2 though). The error message in question may be block size related, what block size did you configure? 8.x defaults to 256KiB while older releases did default to 64KiB, and 256KiB may be too much for the USB (without registry hacks). So try a fallback to 64KiB first. It's an USB DAT, so it's slow as hell anyway. If that works, google for the registry hack to enable larger block sizes and try if they work with SCSI over USB.
Regarding DPX, you know that DPX was an OEM product of BarracudaWare and they offer you to hop over to their Yosemite Server Backup product? I'm not sure it will solve your problem, though, as they are proudly claiming 2008R2 compatibility all over the place, which may (or may not) mean they are 2012R2-challenged as well.
As for complexity and intuitiveness, DP is an enterprise backup system with >20 years of development history. It's so complicated because 90% of it is there to do things you never need in a single server backup (primarily scale to large heterogenous environments), and some of that is there purely for historic reasons and backwards compatibility. That scalability and power comes at a price. But I just saw you already recognized that yourself ;)
Maybe scan the market for other simple one-server backup solutions? I just don't know any, as they always fail to scale before I get involved ;)
HTH,
Andre.