It wasn't an overpayment in my opinion. It was more the fact that they spent that much of their cap to him when they have Marner, Nylander, Matthews all on the team and needing big money
Yes, but there are a couple of ways to look at it.
If you don't sign Tavares, you certainly have enough room for Matthews/Marner/Nylander long term, your team will be built around those guys, and they'll cost around $27m/year combined, add Kadri up front, and you've good core forsure, but I wouldn't necessarily say great.
On the flip side, you sign Tavares, the premier free agent of the last 10 years. You add Kadri, and this is easily one of the best forward groups in the league (likely to be the best within a year or two)., all being reasonably young. As part of the deal, you're now asking the 3 young guns to take a discount... so instead of $27m, you're asking them to be at $24m; knowing that it'll make them cup favourites for at least the next few years.
If they don't "Buy in" to a discount -- you trade them for players who will -- ultimately these guys even though they are young, want to win.