From what I've managed to find, everything points to the T5 being the same thing as a 'Yashica T4 Super D' and a 'Kyocera T Proof'. My list doesn't give me any details on all the accessories, so it's possible that there was a separate and more advanced data back or other accessories (i.e., panorama kit, flash grips, etc.) available for the T4, and rather than design a whole new model as an upgrade, they sold the previous year's model with the back/accessory included and reworked only the logo and packaging and documentation. Kyocera's versions of any Yashica models seem to be simple rebranding and were not much more than logo/color variations from the Yashica's, if anything was even different.
One other thing that isn't in my notes has to do with colors on the T5's. I think I've seen the Kyocera Proof's in silver (or grey or platinum, or similar colors), but if the T5 really was a T4 + XYZ combo, did the newer T5 come in only black, or were there green or safari and/or grey or platinum models too?
This is total conjecture on my part, and it plays into the whole Kyocera-branded camera quandry. I think it's reasonable to assume that the decision to cease the Yashica brand wasn't one that happened overnight. It was likely years in the making, and at least one plan probably wasn't a complete and total shutdown. Tomioka's continuation as it's own division - even after the end of Yashica - gave them a template on how to break down and reincorporate a product line that was more in keeping with their industrial product roots. They could still make lenses and glass for others too, and not have to be a headliner.
SLR/DSLR production would be gone, and so would the remaining Movie/Video line's output. That would mean all of the companion products and accessories could dissapear too.They'd be left with a simpler camera line - licensed, or manufactured - under a brand they were already grooming, and it wouldn't have the demands that the Yashica brand put on them. Tomioka's division took care of the optics, and everything else could be outsourced.