"improvisation occurs only in solo form"

Not true - Jazz comping continues to be improvised and Jazz bands continue to communicate and respond amonsgt each other to changes of dynamic intensity, time and pitch etc

Just check out the way that Chick Corea accompanies ... anyone ... chick is obviously a greeat composer and soloist, but in my opiniion his great strength is his utterly unselfish ability to comp in unceasingly interesting and inspiring ways, feeding his the soloist and respondiing to him simultaneously.

Also check out the way Jazz bass players move smooothly through the chords like Oscar Wilde walking through a crowd.

"a deep, infectious, can't-not-dance rhythmic groove"

This was the case yes, but there were also slower numbers.

"jump blues, rhythm and blues, rock 'n' roll (most notably the truly classic R&B/R&R recorded in New Orleans by Cosimo Matassa). My feeling is that these other forms can also claim direct descent from the founding fathers of jazz."

To some extent yes, but only insofar as they are based on the european construct of I7-IV7-V7 as organized by the classic 12 bar system that blues has come to be known by.

Raw blues had no bars and where there are recordings and we divide up with hindsight, we find 13 bar blues, 16 bar blues, 10 barblues etc.

So the idea of 12 bar blues makes it nearly blues ... add a pinch of european chord theory and the I7 - IV7 - V7 chord foundation makes it less bluesy and veers it in the direction of Jazz.

The more complex the chords, the more Jazzy it becomes, until you reach the point where you are forced to acknowledge multiple key centres, which is where it deviates from blues and becomes something else.

That point is where blues ends and Jazz begins.

Why? what's the difference? where's the dividing line?

It is possible to play Jazz over the I7-IV7-V7 foundation, as there are technically 3 different key centres suggested by 3 dominant chords, but generally in blues the soloist draws from one or maybe two or three scalar resources, all with a root in the same key centre but which still sound good over all three chords - the ambiguous 3rd makes this all the more possible - so while other key centres are suggested by the chords, the improvisation is in just one key so it ain't quite Jazz.

Jazz blues is where the scales used reflect alternative possible key centres and subsequently where the chords are made more complex to the extent that scales used in traditional blues cease to sound right.

The area of overlap is in the classic I7 - IV7 - V7 construct, where you can either improvise using a fixed resource or you can use numerous resources suggesting alternative keys.

But lets say you add a IV7 chord in bar 8 of a twelve bar sequence ... if you continue to draw from a fixed resource with its root in the tonic key centre, you will find that they it doesn't work any more - it isn't enough.

At this point the music can be said to have crossed over into the realm of jazz.

The use of ii m7 chords before each dominant chord, and of diminished sevenths as dominant substitutes further complicates matters as they compel the soloist to alter his key centre for each dominant chord in the sequence.

Funk is more heavily influenced by Jazz than Rhythm and Blues was as while bass lines often suggest a consistent key centre, the chords used in fact define the tonality as being quite complex, in turn requiring the soloist to draw from resources from numerous key centres.

I'm guessing you don't mean R+B as kids know it today, which sounds a million miles from the R+B of the 50's

I haven't knowingly listened to Jump blues so i can't really comment on that.

As for your clip, I hear numerous instruments working in free counterpoint with each other to accompany a loose melody, which suggests that it is Jazz, however, the chord structure seems to be simply either I7-IV7-I7-IV7 continuously, a sustained I7 chord or V7-IV7-I7 and the soloist seems to be drawing from a fixed resource without ever sounding wrong so it is arguably closer to blues.

In fact, I think what we have here is an example of what early jazz sounded like as it first began to seperate from blues to become an artform in its own right.

I hope that that has been helpful and does not appear heated despite the passion and love of the subject with which I wrote it :-)