Lots of questions in this one... I'll do my best to cover them:
Longer cuttings - I know for a fact 6' cuttings (all branches removed) will grow just fine. They need to be planted deep enough that they don't 'wiggle'. Any movement will break off new roots. Not good. The longer cuttings are typically used in situations where you want INSTANT cover. Often in a landscape situation (yard) or a really impatient deer guy. They will look like a semi-mature tree by the end of the 1st growing season. 12' tall with many side branches. I'm going to experiment with HUGE cuttings (up to 20' tall) this weekend. I'll let you know how it goes.
(side note, you can't plant any cutting too deep, as long as a bud or epicormic sprout is exposed to daylight)
Screening with hybrids:
Hybrid willow is the gold standard. They often have multiple 'central leaders'. Their horizontal cover is considerably higher than hybrid poplars. Both require a minimum of 2 rows to create a screen. After the 1st year (assuming 12" cuttings) they will be 10' tall. More rows are better (I don't just say that because I sell them, it's true).
Hybrids are NO replacement for evergreens. Depending on your height requirements, hybrids will create a visual block in 1 - 3 years while the leaves are on. After leaf drop, they will be a slight visual break, but you aren't going to sneak past any deer. I always recommend planting evergreens next to hybrids. You'll get a pretty good screen right away, and an excellent screen 4, 5, 6, years later (when the evergreens kick in).
I have a great tip for hybrid screens - I tried to get pictures of it last year, but we had a near drought. My newly planted screens survived, but only grew about 1/2 as tall as they would normally. I'm going to try again this year. The short story is you plant a hybrid screen, let it grow for a season, and the following spring you bend the trees down and tie them off to one another. Creating 'arches' if you will. This will force the central leader to horizontal (or even inverted) so multiple lateral branches will compete to become the new central leader. The horizontal cover created by doing this is massive. I hope to show you what I mean next summer :)
I would suspect most people would use hybrid willows for a screen. John sells them at bigrocktrees but I would assume they are sold out at this point in the year. If he is north central reforestation sells them at 2-3'.
This is the 1st year that we haven't run out yet. The nursery finally caught up to demand. We harvested approximately 15,000 hybrid willows this year. We have a few left!
I hope that helps a little...
-John