Hi Richard,
I have an impression that most capsizes happen when sailing downwind with too much plate down. In your case, the opposing tide must have made the situation even worse. Regarding diving underneath the inverted boat and pushing the plate up (normally meaning down) – it is exactly what I had to do on a occasion when the plate slid back into case in the process of righting, but I agree it is quite unpleasant and probably risky.
Take a look at this funny video and pay attention to the position of the plate in each case of capsize (470 Croatian sailors):
http://www.youtube.com/watch?v=nDToL593cmU
Best regards,
Mato