I notice that in some turnarounds the II chord is also a dominant 7th as in the progression CMaj7-A7-D7-G7 . Is this just to add interest in the same way that the 6 chord is changed to a dominant 7th (ie. from Am7 to A7) or is there something else going on here ? Also what are the implications for improvising horizontally with the c major scale over this sequence? Should I think in terms of sharping the C over A7 and sharping the F over the D7? I know that whatever sounds good should ultimately dictate what to play but what what is going on in terms of theory?

The first reason that the D7 might be there is to fit in with the tune, probably to incorporate an F sharp.

You can use the C pentatonic scale over the turnaround whether the D is a minor or D7.

You can interchange between Dm7 and D7 when improvising alone but as soon as other musicians are involved they will want to know which one you are playing.

As I always say, any old excuse will do to create a 7th chord, because it opens up a whole new range of possibilities. See episode 12: improvising over 7th chords. You can use...

mixolydian mode
diminished scale
altered scale
upper structures
Lydian dominant scale

but you can't use these horizontally over the whole turnaround, just vertically over the 7th chord.

I can't remember if I've uploaded a pdf file with these examples. If not I'll do so.

Hope this helps