My biggest problem with the offer sheet is that Carolina blew their biggest TDL piece to pick up a giant question mark in Kotkaniemi. A veteran FA or even a small trade for a vet to be a place holder till the TDL or (slightly earlier) probably results in a better player for Carolina.
That's potentially a concern, but there are more ways to skin a cat. Canes still have a 8 picks in this next draft to work with as well as a deep prospect pool so IF they want to make a TDL move, there is plenty of assets to do so.
re: "pick up a veteran FA, this has been covered multiple times. they tried (really went after Saad), but failed so at the point they made the offer-sheet, there weren't any decent FAs left.
1st and 3rd at TDL gets you a more established, and probably better player with multiple prime years left. Boils down to I don't think Kotkaniemi is or will be a big enough difference maker this year compared to what a 1st and 3rd can get at the TDL.
Also, Carolina is not 100% guaranteed to make the playoffs. A good trade deadline pickup could also help swing what I believe will be a very tightly packed Metro division. Caps, Pens and Isles will all most likely make a TDL splash, and those all happen to be three of Carolina's biggest obstacles to getting one of the three auto Metro spots.
Most often, TDL pick-ups are rentals with 1 (or maybe 2) years left, so not sure why you'd assume a 1st and 3rd would get a player with "multiple prime years left". Particularly if, like you said, Caps, Pens, Isles and other teams are also looking to make a splash as that would drive prices up. On the flip side, if Kotkaniemi works out he's a RFA and might possibly be under team control for many years vs. the idea of a "rental" at the deadline.
My point is there are no absolutes here like you are trying to portray. They can still get a TDL guy if they want even after this move. They may be able to get a better player with "multiple prime years left", but most often, deadline deals are soon to be UFAs that teams are moving on from.
Again, I've said from the beginning that I'm not overly fond of the move, but I do at least see some rationale from the Canes management and I don't think the downsides are as big a deal as many are making them out to be (unless of course, the pick is top 5-10).