Ok, I fixed the problem with my HP TouchPad not connecting to adb over usb. The adb driver wasn't installing properly because the TouchPad is not in the list of recognized devices in the android_winusb.inf file (the Moto G was already in there along with a long list of other popular phones).
I had to add this section to the file:
; HP TouchPad
%SingleAdbInterface% = USB_Install, USB\VID_0BB4&PID_6860&REV_0227&MI_01
%CompositeAdbInterface% = USB_Install, USB\VID_0BB4&PID_6860&MI_01
and then update the driver in Device Manager with the TouchPad connected. Make sure to add the new code under the appropriate x86 or amd64 sections depending on whether you are using Win 32-bit or 64-bit (or just add it under both sections). I think I was lucky that I had installed Helium backup on my PC previously, or I might not have had the required Android sdk components
Still have no idea why the network adb backup is so unreliable with the TP, but at least now I can use usb backup. I can't connect adb to my Moto G over the network at all. Android 4.4.4 on the Moto G seems to lack the option to enable network adb that I found in CM 10.1. Don't know whether the option was removed from more recent versions of Android, or they hid it someplace new, or it's just not supported on my non-rooted Moto G.