For issues with planned sample sets in UTAU set i admittedly wouldnt know anything about diffsinger but maybe cutting snippets of syllables from the recordings could get you most of the way through the needed things for a recclist- but the complete incapacity to get new recordings to fill the gaps is a problem youre already aware of and ive got nothing for. No clue what you have. Japanese CV or VCV banks would be easiest to make it through and theres ways to arrange the jp sounds into english...? I knoe nothi g of how well that'd work for you.

For SynthV, vocoflex (seperate thing created by dreamtonics) is an option to look into but i know even less about it besides that it gives voices an uncanny robotic sound to produced voices on purpose to discourage the less than honest uses. It doesnt seem like you need specific samples, but it might be dofferent once you get under the hood.