I am really not convinced (and I have a biais pro Euro skaters in general) he has bad offensive stats overall in his D+2. If he turns into a bottom 6 forward, is it really what you should get for your top scorer ?
First of all, yes, Heineman doesn't have Pettersson-like stats in the SHL. But given the fact that he plays less than 12 minutes a night with very little to no powerplay time, on what is essentially Leksand's fourth line, his 16 points in 36 games are actually really impressive considering he's playing in a league that many (including myself) would consider as the fourth-best in the world.
If you are not convinced, by all means check his stats out for yourself if you want to know more, directly from the SHL's own website.
Statistik spelare 2021/2022 - SHL.se
As for the second part of your post, my answer is simple, you can't just take one of the multiple pieces we got in exchange for Toffoli and evaluate it on its own without taking into consideration the added value that the other elements bring to this trade.
Is Heineman straight-up for Toffoli a good trade? Of course not, we trade the better player AND take on much more risk given that one is still unproven.
But is Heineman, a conditional first-round pick in 2022 (most probably 18-24, but could be 2023 first+4th) and another 5th-round pick good value to get for Toffoli in a package? Seeing as we're getting a strong "B" type of prospect in Heineman, a chance at getting another "A/B+" type of prospect with the 2022 first AND more picks still? Heck yeah it's good trade value. Even if we lost arguably our best forward offensively this season.
Now to the last matter at hand, namely Heineman himself as a prospect and his place in our pipeline. In that regard, I would actually rate him a bit higher than Luke Tuch (not as physical/tough, but better skills overall and smarter on the ice) and Riley Kidney (Kidney has better vision and puck skills but Heineman is better two-way, more physical, faster and plays a more "pro-type" game).
Also, I would put Heineman on a similar-ish level as a guy like Mysak, as he is probably safer than him but maybe doesn't have the same top-end potential.
Farrell and Roy I would rate a bit higher though. The offensive potential is sky-high in their case. But so is their bust potential unfortunately.
So yeah, that's it for me. Good day and cheers.