Huh. Got to the exact same point. Except now instead of giving me the message about "extras+info", the "Next" button on the "Almost done..." screen does... absolutely nothing.
So I rebooted, and... I'm in! Go figure.
Things I did differently this time:
- After the 8.1 update, made sure to let all apps finish updating before launching the Insider app to enable the 10 build.
- Selected the Fast Ring.
- Connected to a Wi-Fi network after hard-resetting when the phone was in the reboot loop after installing 10.