Simpson strong tie 1 1/2" steel nails, or Simpson # 9 x 1-1/2 in. 1/4-Hex Drive, Strong-Drive SD Connectors
yes you could do that.
Ties can be drilled out to whatever fastener you use. I think the size in the tie is for nails, but I have drilled out the ties many times
I used #8x1 3/4” screws
Pocket screws are better for pocket holes, because they come in different sizes for different size boards.
I believe they do, I bought some there before.
I found that drilling the holes to fit the screws works well, as long as the screw is not too thick
If you are doing this without permits, you could probably add to a 2x3 using plates on both sides of the 2x3, using pan head screws instead of nails. I would suggest using drywall screws instead of drywall nails. Another trick would be to cut longer pieces instead of using "stubs", and staggering the 2x3s for greater strength. This would give greater strength to the wall.
Not suggested; chance that tie would pull out of concrete. I would use the fitting made for this purpose.