1) sweating is an individual thing. having your head and feet not in the dome is counterproductive for sweating.
the fir sauna boxes work on a combination of ambient heat, like a regular sauna, and the fir emitters. the ambient heat in a fir sauna is much lower than a regular sauna, but it is still important. with your head uncovered you will be less likely to reach 'critical mass' where your body begins to sweat.
the best fir saunas in my opinion are the closed boxes. the portable tent units are not strong enough. in a closed box with strong enough emitters you should be able to get a very good sweat going.
you can even build one yourself if you have a very small closet to mount the emitters.
2) in my opinion yes. others may disagree and people like james wilson feel that the benefits are mainly reaped by inducing the light frequencies, but i totally disagree with that. i think it is 99 percent about sweating out toxins.
3) doubtful. fir mimics a subset of natural radiation from the sun.
4) i doubt that there would be any interaction with the Colloidal Silver . the worst reaction is if you are already dehydrated and attempt to sweat, or you are intoxicated with alcohol and begin to sweat. some people with heart problems or high blood pressure are not advised to take sauna
hydration is a big part of this equation. you must be properly hydrated before you attempt a sauna.