Hey John-
Hope I can answer at least part of your question. In terms of the chord cycle of the 12-bar blues, I have a lesson on that at this link:
http://www.guitartricks.com/lesson.php?input=10765&s_id=379 The lesson is oriented towards a fingerstyle technique, but the concept of the chord cycle carries over a variety of techniques, from fingerstyle to electric rhythm blues. Pay attention to the pattern of the chord changes in this lesson, in the key of E. How many measures of each chord, etc.
You can then transpose that same pattern to other keys (you mentioned G,C, and D etc.) - keeping the same number of measures, the same relationship of chord changes. That's what make the 12-bar blues predicatable, and therefore, easy to jam on because you can easily predict what is going to happen next.
What chords to use in each key for the basic 12-bar blues? The "one", the "four", and the "five". For example, in the key of E, it's E (1) , A (4), and B (5). In the key of G, it's G (1), C (4), and D (5). Try it - I bet you'll be able to hear it.
As far as riffing and improv, the short answer (if you know your pentatonic scale), is to start your scale on the same root note as the blues key you are accompanying is in. For instance, a blues chord cycle in the key of E would be accompanied by a pentatonic scale starting on the note "E".
Hope this helps!
Best wishes - Lisa McCormick, Guitar Tricks Instructor