I can't see Buffalo giving up Philly's 2nd in the deal, but I think they would move their own second.

CGY could get a healthy amount more in value by trading Tanev at the deadline. Dmen of his calibre/impact pretty handily fetch a 1st ++. Two second rounders aren’t going to move the needle.

Overvalued the old Tanev as much as I like him

In general I would agree with you, but here term(1 year until UFA) and injury history play a part. Tanev had a hard time staying healthy last season, he missed time, and was playing through one thing or another for most of it. He missed quite a few practices as a result.

At this point Tanev is a player I would prefer the Flames to re-sign for a few more seasons.

Take a look at previous deadline prices for dmen in the final year of contract. Renting a top 4 shutdown dman on an expiring contract is very expensive. More than two 2nd rounders expensive.