1) Well, it depends on what you have right now, if it is gas, you should go for gas. It got to be gas, but get it checked first. you may ask the agents.
2) For my knowledge, KYB is the best as they are the largest shock manufacturer and the market leader in JDM. But Tokico too is equally good.
3) Its quite unusual but as you have already mentioned, there are many parameters in to it.
4) Oil is good in comfort and gas for better handle. However, for my knowledge, most of the cars have gas shocks nowadays.