I would say you are correct due to Millwall Blackburn Coventry all playing each other. Take out WBA as perhaps being 5th, then I can’t see how 2 of the 3 above can hit 75 points, which with your goal difference is what they would need.
I could be wrong though, but for me 74 with a plus 28 GD is uncatchable.
There may be a way ?
9 point gap on 7th.
If West Brom beat Sheffield United in their game in hand, it's 8 points.
We're on 71 points right now, the max we can achieved is 83 points.
Current points and maximum totals possible:
Blackburn - 63 points, max of 78 points
Millwall - 65, max of 77.
West Brom - 60, max of 75.
Coventry - 62, max of 74.
Preston - 62, max of 74.
Sunderland - 61, max of 73.
Norwich- 61, max of 73.
Blackburn play Coventry on Wednesday, so one of them is dropping points.
If Blackburn win, Coventry can't catch us (bar a huge goal swing and 4 straight Boro defeats).
Blackburn then play Preston, who can't catch us unless they get 10 points from 12.
They also play Luton and Millwall.
West Brom play Sunderland and Norwich, so whatever those results, it ensures at least one of them can't catch us.
Sunderland also play Preston, so that's another guarantee that one of them can't catch us.
Because of all the head to head fixtures, it basically means that we're already in the play-offs.
For most of the teams outside of the play-offs to make it, it would see Blackburn dropping out before us.
Most of them will also knock another chaser out of contention along the way.
Regardless of results elsewhere though, 4 points from the final 12 available would mathematically guarantee us a play-off spot.
3 would realistically do so because of the goal difference.