Laine was signed after that disastrous 2018-19 season, where he really was an absolute disaster. 9 goals from December 1st onward, -24 on the year (the nearest teammate was -12), that's more than just a bad half season, that's a bad season. Laine's torrid scoring in November actually obscures how much he looked like a zombie for the first two months of the season. No that's not better than Connor, who signed at the same time after a strong year.
Yeah, as said, the latter half of 18-19 was bad. Even horrible for Laine's high standards.
But even with his bad season 18-19, Laine was just 3rd on 5-on-5 goal differential 16-20 after Ehlers and Scheifele (and better than Wheeler). And if you omit that season away as an anomaly, Laine did lead the entire Jets team on 5-on-5 goal differential (a.k.a. winning) for 16-18 + 19-20. He definitely leaves Wheeler and Connor biting the dust on that comparison.
Laine defends by playing solid, quiet positional play with his high IQ. He often takes away some high quality chances by backchecking. Nothing fancy, nothing to show in highlights and nothing to notice if you only follow the puck. But Laine's defensive play is not actually bad. It's just not excellent either. He is better defensively than Connor and more responsible defensively than Wheeler & Scheifele who way too often to over-commit themselves to offensive zone. However, I expect Laine to be one of the worst forwards defensively for CBJ as that's like the thing with Blue Jackets.
PS raw +/- is hardly usable stat as it includes only minuses from PP and only plus points from PK. Also empty netters count as full plus and minuses screwing those stats even further. Therefore I suggest using just 5-on-5 goal differential or "5-on-5 +/-".
PPS As you did bring up Laine was worse than aging Wheeler last season playing 1st line, that's not exactly true. When Laine played with Scheifele w/o Wheeler, they were doing more than fine: +38, -33 on 5-on-5 game. When Wheeler played with Scheifele w/o Laine, they had +18, -16 which is basically the same, only slightly worse.
Last season Laine got 0.51 5-on-5 PPG, Wheeler 0.44. Not a huge difference, but anyway you can hardly say he was worse than aging Wheeler. He was not. Wheeler just started his 8.25M AAV, 5 year deal which I expect to be horrible value towards the end of the 5 year period but anyway.
All in all, 8M would be somewhat lowballing with Laine (unless it's short term), but let's see how the season is going on. Also covid & low CAP has already made things worse so it might be that 8M eventually is what they settle on with. Without covid, I think something in line of 9-10M would be the actual AAV.