How important is it to play 'Chord Tones' in Jazz improvisation?
Is this question pointed more toward creating basslines to support the melody and other soloists?
Very.
b) To 'mix & match' with the chord scale notes and add passing notes?
Yes. It's a good idea to place chord tones on the stronger beats and use passing tones on weaker beats - especially when playing a supporting bass line. Of course there are exceptions. When soloing, one can use Pentatonic Scales, very effectively.
c) In a chord sequence--example a Blues---is it better to use the dominent 7th than the major chord?
Major Seventh chords are illegal in "Blues"
Blues forms use "Dominant Seventh Sounding Chords". That's to say that not all of the "Dominant Seventh Sounding Chords" traditionally resolve as a Dominant Seventh.
There is what is called the "Bebop Dominant Scale" that can be used over a "Dominant Seventh (Sounding) Chord."
Example: Over a C7 chord, use this collection of notes - C, D, E, F, G, A, Bb, B, C. It has BOTH a Minor Seventh and Major Seventh.
The use of the Blues Scale is also a great way to play in the Blues genre.
There are also the Minor Blues Forms, you may like to explore.
There are MANY ways to go.
Look at as many transcribed bass lines and solos, as you can - they are everywhere on the internet. Then start your own transcribing. You will learn faster, better and deeper by doing transcribing.