RAD questions

I have a 300 square foot deck. I have no idea how old it is. The house is 20 years old and I moved in 3 years ago. It was already weathered and I doubt if it had ever been treated. I sprayed Olympic Maximum transparent sealer on it two years ago. I pretty much decided to strip it and brighten it with RAD products and then use their Natural semitransparent stain. I am in western North Carolina. The deck gets a lot of sun but the south side is almost always in shade.

Questions: 1. I just painted the 6x6 posts and railing with Superdeck solid. Will the RAD stripper take that off if I accidentally get some on there?

2. Would 2 gallons of stain cover it with two coats? I don't see where I can buy just one 2.5 gallon container on the RAD website.

Bonus questions: Is it common practice to put the first deck board partially under the house siding (see picture). I think it would be a PITA if it ever needed to be replaced. Pine needles build up between the first and second boards and it is impossible to get them out without removing the second board. I will do that this time. Is there something I can treat the first board with - like some borate or copper solution - while I have it exposed? I will apply the stain to the top of the board.