According to some chatter on Nikonians and DPreview, the D800/810 will sync up to 1/800 with essentially a 20% crop of the bottom of the picture, so while you won't get the full field of view you'll get a bitter than DX crop of a full frame shot. At 1/400, it only crops about 10%. One poster on Nikonians illustrated with cropped shots of a white background shot with a D800 at 1/400, 1/720 and 1/800.
All fixed. There's a paper collector on the flood sensor that stays wet. As long as it's wet, the light stays on and the strobe won't function. Thanks to Bob at Southern Nikonos, I dried it out and everything works.
Thanks to Bob Warkentin at Southern Nikonos Repair, it's all fixed. He graciously sent me instructions on removing the top cover and drying out the leak sensor, which seemed to be the only issue, and now the strobe functions normally. Removed the hardened old o-rings and installed a fresh set and hopefully the problem won't be repeated.