#9 x 2-1/2 in. Star Drive Bugle Head R4 Multi-Purpose Screw (100-per Pack)

Not sure, but they include a quality torx bit in the package.

Yes, but I would choose to use a screw that has a head that will NOT chew it's way down through the wood. Screws without that chewing feature will tend to pull two pieces of wood more tightly together. The GRK fasteners seem to result in loosely-attached joins.

Absolutely. I use them for outdoor furniture, decks and fences. They never stain the wood like some types of screws.

No, these are wood screws.

Yes they work great, but ideal depends on your project. Remember screws are not designed for framing, they are a harder metal than nails and are somewhat brittle and can break easily, they will not flex as a nail would. But as a general fastener they do work very well, they start very easy and have a very good grip. I have used them to pull a twisted or warped 2×4 straight followed with a couple 3 1/2 framing nails.

I thought they were #8 but if the box says #9 then thats what they are.

It can be used for any wood to wood fastening task, but I would suggest a somewhat shorter length unless there is some factor I am unaware of in your situation.

I wouldn't use them for that. The heads are designed to chew through the wood. You want a pocket-hole wood screw with a flat head that will bull the two wood pieces tightly together without damaging the wood. You should buy pocket-hold screws instead.