You have $5.5M cap space, and Tanev played great for the Pens. Why are you paying to move him out?

I can see paying Seattle to take Matheson instead of Tanev, but if you don't, the Tanev "problem" will solve itself.

What's wrong with Guentzel-Crosby-Kapanen, Zucker-Malkin-Rust, McCann-Blueger-Tanev?

I mean he really doesn't unless you think Marino is signing for 4.75....which I don't think is going to happen, especially on a 5 year deal which makes no sense for the penguins.
He either signs 2 years, or 8 is my guess. And either way it's for more than 4.75. I'd guess he'll get 6 range. Blueger and Zar should have enough cap space though to keep Tanev.

Why are they paying a primarily fourth line player $3.5M?

Even if Marino makes 6m, they don't need to move Tanev and probably don't want to.
6 or just under seems fair for 5-6 years, but much less if its a two year. Maybe like less than Serg money....around 4.5

He plays third line minutes
Big part of the PK. Sure hes probably making 1m or so too much. But they really like him and he was quite good.
UFA prices and all

Because there was a 5 team bidding war for him. Why are you paying to move him out when you have more than enough cap space for him (and the previously mentioned teams want him thing)?

Buddy look

1. Brandon Tanev is going to be protected
2. Tanev got a 6 x 3.5. The money is okay, but the years isn't.He is earning Bryan Rust money.
3. Tanev is fast, and furious, he is Carl Hagelin but with some skill.
4. Hes the Penguins fastest player.
They won't get rid of him.