Why does Vmc (indicated) decrese with an increase in density altitude?
main driver is thrust. However, be aware of the differences between a normally aspirated and turbo/super-charged installation
max power on the operating engine and 3-5* bank away from the failed engine
You might be confusing things a little. Vmca usually involves a restriction on the OEM to limit bank to 5° so, unless you have specfic Type information that a lesser bank angle was used, presume 5°.
For OEI climb, the best performance generally is somewhere around 3°. This will have little relevance to Vmca.
full rudder , opposite aileron and the stick on the back stops with the stall warning going before I lost directional control
obviously this happened a long time ago .. but how does one spell "spin" ? Unfortunately, the military lost a lot of folk during training mishaps rather than due to combat.
Sounds like you are fortunate to be with us still and be able to tell the tale.