I think I said before that anything past 10 minutes is not worth doing since the fog starts to develop. I was hoping you'd try 10 minutes at a higher temperature like 75 degrees, say? Instead of cold at higher time. It appears to me that the increased temperature gets more WHAM at the beginning. It's like you CAN'T make up for less active developer by developing for longer times--there's a certain activity level you need.
Before you go with the solvants, there's D-11 and D-8 developers also--much stronger than d-19. You can also goose d-19 by adding more carbonate or caustic soda to make it more powerful--or use it at a hotter temperature. Note that no reversal labs use 68 degrees..they run higher and sometimes very high temperatures. Higher temperature developer for a shorter time does not behave the same as the savve developer when it's cold for a longer time. When you're doing "extreme" developing like this, the "reciprocity" of time and temperature seems to get left behind--like film for very long exposures..it acts differently.
Before you try to do solvant blindy in the first dev, I'd suggest you do it with ferricyanide bleach--you can take your current results and slowly dissolve the silver on them to "develop by inspection". It works and it's more controllable when you can see what you're doing. That hypo in the develeloper is hard to guage since there's 2 things going on at once. You won't lose any more silver than the hypo in the developer will take away--nothing to lose.
Anyways--I still firmly believe that hypo in the developer is unnecessary and therefore should be avoided.
I believe that stronger, more active developer is the way to go--you can see from your results--more power is the key--you just need more horsepower in that 10 minutes--higher temperature? more carbonate? D-11?--you have the luxury of time to develop--unlike labs that develop MILES of film--their processes must be set up for minimum time. You need a process that takes like 10 minutes to promote uniformity and to lessen the effects of filling/emptying. Even 6 minutes is pushing it pretty quick...that's why it's the starting point for this type of work.