SANDIN:
Dubas has done Sandin a favour and got him more playing time (the point of contention in his contract hold up). That's all it is. The Capital's LD side is wide open and he'll get to play with someone like Carlson or Jensen or van Riemsdyk under almost no pressure to win (just feed Ovi's record chase). While Carlson is out injured and Gustafsson is gone, they have no more PP options. A bit of a trickier pass for a lefty to feed Ovechkin's office, but Sandin can figure it out.
For the Leafs, they've clearly indicated they can't wait for 100+ more games for Sandin to reach the level they need today. Rielly's re-signing and the myriad of motivations behind it include it taking too much time for Sandin to fill that spot (and maybe too much cap space while waiting).
ENGVALL:
Similar course in a front office favoured project (Marlies champ) that just ran out of reasonable time within their cap framework. You can get his on-ice results (in a different way + more) by paying less; Engvall had lots of chances to demonstrate he could take that opportunity but he just didn't. Maybe the best 4th line in the world can teach him how to forecheck more physically. Engvall has the body of Optimus Prime but unfortunately also has with the processor speed of a Pentium 2 paired with the instincts of a perimeter playmaker.
Overall:
I really liked both players, but the shine starts to wear off when they climb near 3M, which is what next contracts would've been hovering around. Like the Kapanen/Johnsson/Dermott trades, it feels like a small step back when viewing these moves in isolation, and pretty ruthless from Dubas, but he has created room to change the identity and complexion of the bottom half of the roster. More will to support the skill. We've seen Matthews get fired up from Simmonds or Matt Martin shenanigans; I don't recall anyone on the bench being particularly excited for Engvall's minutes.