1. Do you have access from the back of the wall, or are you going to try this from the front? If from the front, you'll need a remodel plate to cover the larger hole needed to make the connections.
2. Plumbers go to school for years...hard to cover everything in a few words. Again, if you don't have access either from below or behind where the drain goes, it's much harder to get it right. What do you have?
3. Follow the manufactuer's instructions, but my preference, when allowed by the manufacturer, is to use a mortar bed. Some piles of sand mix (mostly sand with some portland cement - not runny, more like wet beach sand) strategically placed, then the tub smushed into it to get it level at the desired height.
4. Where are these holes that need to be drilled? My preference would not to use galvanized, but maybe brass or SS. If it is a sheet metal screw, the size depends on the screw (well, it does for any screw fastener). Too big, not enough to bite, too little, you can't screw it in. Normally, on a self-threading screw, the pilot hole should be about the diameter of the shank of the screw so most of the thread cuts into the material.
5. No...that stuff requires way to high of a heat, and will give you a mess. You need plumber's solder and some flux. If you haven't soldered copper/brass much, you need to practice a little first before you attack the real task.