Awesome idea. Would love to do this for football. The only thing is, hopefully we could get enough people to be interested. It took us like, 3 months to fill the decade and draft leagues, and that's with some owners taking multiple teams. Yikes! That said, I played in a progressive league for the NBA sim, and I think some of what was used there, could apply here. I'll throw these ideas out and see if any of them stick.
In regards to questions 1 and 2: I suggest, instead of having it be strictly progressive or regressive, let people choose what season of the player/defense/line, etc. that they want, regardless of whether it is forward or backward in time. Have the draft be an all-time draft, not just from specific years. Every player, defense, o-line, special team, kicker, everyone is exclusive to the owner that drafts them (this would make the draft longer since every position on the roster is being drafted). The owner can then choose any season of that player/defense/line, etc. but it can only be used once. So for the next season of our league, if they hold on to that player, they have to use a different season. It can be any other season, regardless if it came before or after the season that was used originally (hope this is making sense). After the first season of the league, everyone posts their keepers and cuts, and then another draft is held before the new season, and anyone who wasn't originally drafted, plus cut players, would be available to draft. If a cut player is drafted by another team, the same rule applies that they have to use a different season from what was used already.
If you do it like this, that eliminates questions 3 and 4. I hope what I wrote makes sense.