Sorry, no help on the bottle. When I did the switch in my 98 I scrounged an extra fog light switch from the junk yard and mounted it next to the existing fog light switch. Looked like another o.e. switch because it was. Nobody ever caught on
Do a cutout in the rear seat bottom foam, should keep the bottle insulated well and will be undetectable - plus there are plenty of places to run the lines (out the ABS sensor holes, through the frame, etc.).
Has to be somewhere you have easy access to the bottle. With one that small youre gunna be filling it regularly. I would say underneath the mat in the trunk. You might even be able to fit that under the spare.