1. Yes. Amp is in floor on passenger seat side.
2. Most modern head units have a pre-amp out connector. That is, one that is set to drive an amp, not a speaker. Doesn't yours have a pre-amp out? The guy who sold you your HU should know.
3. Electronics, like A/C, works like magick. Stay as close to OEM specs as possible.
4. Remember "search" is your friend.