Colorado traded players to get players. After #1 (McDavid), the Oilers have 3 centres who are better than Middelstadt including the one they just acquired. If anything we are out "arms racing" the Avalanche, we got the better player. Walker is not a needle mover.
You are probably alone in your opinion on Tanev. He's a good one way d man that has taken a bit of a step back this year. I think you would struggle to find a single other person that would describe him as the best available player at the deadline.
I think Vegas's defense is already solid and already efficient through three pairs. I don't think the impact there will be material, and I think Hanifin is massively overrated. This could be a blessing in disguise, because McDavid absolutely abused him in the playoffs just two years ago. Hopefully he brings the same level of impact. Don't think that Hanifin makes a big difference to a team that has been playing at a 25th place level since November.