I'm in the same boat. I made an update package and installed it, but when I wasn't able to talk to the Solo over Wi-Fi and the USB serial interface on the accessory connector. I took the Solo apart and manually went through the factory reset procedure on the SD card (following the steps in "init" in the initrd).
Now that I have it open (and an image of the SD card), I can be more adventurous.
I think my issue was with the modules. When I build the modules, some of the supporting files ( modules.alias, modules.symbols, etc) differed from the files that I pulled from Solo's 3dr-solo-imx6solo-3dr-1080p.squashfs.
I'm going to try building the USB CDC driver into the kernel to see if that helps talking to it without Wi-Fi. I assume I'm building/installing backports incorrectly.