My PBP starts up like a 1977 Chevy Nova

[TRS-80]

I'm outta ideas here. It crashes too much to be usable. I first received this thing in April and I have been poking at it since I got it, but it has never worked.

This level of frustration is likely to put you off from entire companies or perhaps even categories of products (SBC in general)?  I applaud you for sticking with it.  OTOH, if you do ever get it working, you will obtain a (well deserved, IMO) sense of accomplishment. 

Could the emmc be bad? That seems unlikely to me because I dd doesn't throw any errors when I write an image to it with the usb adapter.

I would check, again just in the name of thorough troubleshooting, especially since it seems you have tried everything else.  If you don't want to spend the money/time to purchase another, and are willing to share your location, maybe (slim chance) someone can meet you to help (if they have a spare)?

Above is one big reason to become friendly with local hacker spaces, LUGs, HAM clubs, etc...