OK I asked my mum (who is a very experienced Cat Lady) about your situation and she thinks that your problem could well be mixing the biscuits with the meat/wet food. She has a 19 yr old tom who while otherwise healthy for his age, started bein sick after every meal (undigested food with no unpleasant smell, just like yours) and she now doesn't give him the biccies at the same time as the wet food or other food. The biscuits swell in the stomach with water and digest at a different rate to meat and she says that for Bogart it works better to feed him exclusively wet food for breakfast, then leave down the biccies during the day she has to go out and then feed him wet only for tea.... whereas the others in her tribe can have biccies and wet food at the same time for all their meals. She says he's all but stopped doing it with this routine.
I think a lot of ppl like to mix the biscuits with the meat as it looks like more appetising a meal to us... Mum says "yes but you don't mix your pudding with your meat, or your cereal with your toast". I guess she has a point.
She also suggested to try doing this with the Hills for a week, then with the JWB for a week, so you can see if there's any difference and if either one of those are causing the problems.
Hope it helps x