I guess I can't edit my premature post above.
Anyway, this is weird. I turned frig off on control, compressor went off. Then tried forced defrost again, compressor went off! So then I tried the internal service diagnostic (position #1) for the thermostat, that now gives an "S" not a "O"!!
So maybe the board is going bad, or maybe it rebooted itself (I don't know if defrosting it manually had some effect, or just from powering off, or both) but it all indicates working normally now. I won't know for sure until it remains frost free - how long would that be, at least a few days? longer? I guess I can check if the heater comes on in forced defrost mode, but that requires getting behind the panel again, easier to just wait and cross my fingers.
As far as original symptoms, freezer was working fine, frig warm. Evaporator fan and damper working normally. Excessive frost buildup.
Thanks for the responses!