Let's talk offense, though. I saw probably 30 Griffins games scattered across the season. I'd say Mantha played on the power play for 10 of them, and mostly at the beginning of the year. I estimate he played on the first power play unit for 3 of them (mostly at the end, when Pulkkinen was gone).
Assuming that's a decent sample (it should be), then .53 P/G is actually pretty darn good at even strength. Especially if we look at the fact that Mantha, in his scant PP time, managed to score 6 PPG, which was good enough for 3rd on the team. Pulkkinen only had 10, and Miele had 8. To me, that implies that Mantha could have scored more power play goals if he had gotten the chance to. After all, Mantha wasn't even playing on the same unit as Pulkkinen and Miele most of the time! and he surely got far fewer overall PP minutes, probably less than half as much as the others.
I think this suggests that Mantha didn't struggle offensively as much as we might think. In particular, I think if Mantha gets those PP minutes and hits 20 goals (and I think he would have!), this narrative doesn't get turned around on him this way. But because he hasn't done a good job at playing a two-way type limited role, it's somehow being turned into this idea that all aspects of Mantha's game are equally limited. and that is absolutely not true. At least offensively, he's fine.