Maybe you should ask Vinz for that, i know he built some in the past.
An Example: http://www.taschenlampen-forum.de/drop-ins/20028-uv-365nm-d26-einsatz-lz1-10u600-md26e-uv.html
The difference between high and turbo is very small for the eye. A turbo mode with 1600lm seems to be twice as bright than the high with 400lm. That is why the turbo looks not more than twice as bright than high. Only ca. 50% brighter.
The Nichia has a really high Vf and the driver is optimized for 2cells (or 3 CR123). The bad regulation with one cell is the result. No one said that there would be a regulation with only one cell.
Yes, your battery is dead! Please don't charge it anymore, you can harm yourself or burn your house down.
It should be deffective, contact Fenix for a repair.
All cells with xxxfire are crap ans may be dangerous.
High quality cells are Keeppower, Enerpower, Xtar, Efest, 4sevens, Tensai.
Do a search, there are more than enough topics about 26650 cells.