I'm guessing you sorted this or got on with your life as it was a while ago! CardDAV is definitely a little idiosyncratic! I haven't found that not implementing me-card has ever caused ios contacts to crash though.

One frustrating thing about iOS and the contacts app worth being aware of is that it syncs when it wants to and pulling down to re-fresh does nothing.

If you're still having issues implementing this I'd suggest using Charles proxy with SSL enabled to snoop on the comms. You can connect your phone to it easily enough.

To debug you could sign up for a trial at Contactzilla (this is my product, heads up) and there you can snoop on the process. If you get stuck feel free to shoot me a line.