I finally got it to downgrade via the serial console. This probably would have worked from the beginning, but I could not connect to the router using the standard 115200 8N1 console config.
Ended up working at 2400 8N1 - go figure. Not sure what sparked me to try this...
I had another RB433 that I had put 4.3 on and was able to MAC Winbox into it from Ether1 only. This however was not the case with the 2nd unit that I put 4.3 on.
By the way, my R52H wifi interface was working fine on 4.3 - but because of the way I have my router configured and locked down, I was unable to access any of the mgmt features.
Guess this was a good test of the security I had setup.
Hope this post helps someone else with the same issues.