Would it be so promoters doing early team building have certainty about riders averages? [Which would suggest a fairly high degree of confidence about next season's league format.]

I don't understand why they'd not count the last 2 fixtures. I'm guessing it's to stop riders trying to drop their averages once the play-offs start? But even if the matches took place before the play-offs, neither club could make the top 4. So what difference would it make if they rode before or after the play-offs? This is just an assumption obviously

Well they are still to publish the final green sheet declarations.

would think it more likely to lead to some average manipulation toward the end of the season depending on the Greensheets.

Richard Lawson, Chris Harris, Sam Masters, Dan Thompson & Cooper Rushen.

All teams build to same points total. If a team uses more points on its heat leaders then it may well be stronger in that department but potentially weaker lower down as a consequence.