Welcome!
I"m fairly new here myself. I have a 99 SL2 automatic. I'll try to answer most of your questions.
Here is my HAI thread with some pictures. There isn't an official "howto" but if you're handy, you can make things up as you go.
HAI Thread
You can check out my gas log to see what I have done. Its not much ... inflate tires, smooth hub caps, HAI ... planning on belly pan.
Tires ... close to max pressure on sidewall helps a little but hard to quantify. My fronts are 35/35 max and rears are 35/44 max. The tires are rated to run safely up to the sidewall pressure.
Coroplast hub caps here
I'm sure coasting in neutral is probably illegal in all 50 states. Can't help you here b/c I don't know much about manual trannies but try not to get a ticket.