We can dispense with some of the items for now but as to the extent of the people and the time of their arrival there are a few points to be made.

Assuming the arrival of PIE in the 6th millennium BC is about 3000 years too late. It either arrives in the 9th millennium or we are confronted by an invisible invasion again. There is just no major population moves from this point in time that we can find. We can leave this as the authors are, as you say, making a cautious approach, but it is a point of consideration.

I do not view Celtic and its daughters as trade languages but rather coming form a culture with a strong interest in trade and trade routes. The natural avenues of expansion are river networks.

Please don’t take this personally but very, very few Native Americans were Nomadic until the arrival of the horse, post contact. Nor were many hunter/gatherers. Except in the far north and in the Great Basin area they were sedentary farmers living in villages and planting a variety of crops. The particular people I had in mind were the Objwe. They lived between the Great Planes along the Great Lakes to the St. Lawrence Valley. They made copper tools and kept records on wood and bark using a pictographic form of writing. Not Iron Age to be sure but they also sat a the hub of a vast trade network. As I hope you see, there are some parallels to be drawn.

The Geographic are is that primarily north of the Alps and following major river networks. Iberia ( Targus Valley) and the cost of France were linked by sea even in Paleolithic times showing it must have been a part of an overall network of contacts or trade. The Po Valley also is accessible in the same way, by rivers and valleys.

I am failing to fallow you on the proposed times and roots for the development of the other PIE languages. Are you saying that what was spoken in Central Europe is something similar to Celtic and from this root developed Italic and German and so on? If so, I think that is overly complicating matters.

Is it your thought then that P-Celtic and Q-Celtic are also offshoots of this language at different times?