A note on your first question that kind of goes along with lalimacefolle's post, a surefire trick for what scale to use is to write the notes of all the chords in your progression, and knock out the duplicates, for D-G-A, you have d-f#-a, G, g-b-d, A, a-c#-e, put the notes in order, and it looks amazingly like a D Maj scale, now add in any other notes you might want to play, like the f natural from the D blues scale, as far as double stops and 2 note bends go, a general principle is that your are using part of the chord you are playing over, or maybe adding some color to it by adding in a 7th or whatever sounds good to you, with the 2note bends, you are alot of times bending into the chord, ie... fretting the e and bending the b to c# for the A chord, or bend the b&g to c#&a, everything depends on what you want to hear, let your ears tell your fingers what to play, not the other way around....