"I agree with nearly everything you're saying. Except, we have seen several players who have been drafted only because they were committed to a JUCO. In other words they would not have been drafted at all had they been insistant on attending a 4 year school. For these players, waiting until after the draft to commit to a JUCO could get them undrafted."
In my experience, that would seem to be true for players that were pretty marginal prospects.
"While it is rare, we know of one D&F who signed after his first year at a JUCO (with the club that drafted him) for late 1st round money."
It has happened & will happen more & more as time goes by. The reason being that as the attempt at "slotting" continues, more people will find this a viable option for them.