I know this is a bit mental as far as play-offs go, but these issues can be solved.
Say for example, in the middle eight, the teams finishing 9th, 10th, 11th and 12th in the Premiership start with 12, 10, 8 and 6 points respectively. The teams finishing 1st, 2nd, 3rd and 4th in the Championship starting with 4,3,2 and 1 points. This gives the bottom teams in the Premiership and the top of the Championship something to play for in the first 22.
In terms of the top of the bottom eight and the bottom of the top eight, have them play-off against the teams who finish 4th and 5th in the middle eight respectively for a position in the following seasons Premiership.
There are of course flaws with this. It could mean the team who finish 8th after 22 games are the only team relegated and the team who finish 5th in the Championship are the only team promoted. It would however solve the imbalance of the split and give teams more to play for throughout the entire season.
Is this too mental?