NHL teams just do not value Dal Colle as much as they have in the past, simple as that. He may have gone 5th in 2014, but the other players in that draft have progressed while he has stagnated. Ekblad, Reinhart, Draisaitl, Bennett, Ehlers, and Virtanen have all progressed to the NHL level and all are putting up great numbers for their age (Virtanen not as much, still though), While Nylander is lighting up the AHL. Don't get me wrong Dal Colle is still a GREAT prospect, but he in my opinion is no where near the level Bennett is, or most of the top 10 in that draft for that matter.
In regards to Hamonic and how he fits in to all of this ... he evens out the trade a little, but to me not enough. Sure he's a lot better and younger than Wideman, but that doesn't balance out the scales enough in this trade. I just don't buy into the whole Hamonic hype. Is he a top 4 defenceman, sure, but a top 4 defenceman and a 'pretty' good prospect is not enough for a young number 2 centre with serious number one centre upside. That's my take on this trade anyway.

Would it be better if the islander were to add a 1st and a second or would there be a better option?

That would probably do it, but at that point you're just looking to make a blockbuster, if I was NYI GM why bother rocking the boat like that when your a serious Cup Contender in the East already?

I actually don't think the deal is that bad as is. At first glance I thought maybe the Isles were giving up too much, but I haven't really kept up with Dal Colle's development. Maybe boucie is right in terms of Bennett being far and away the best player, but Hamonic is a pretty huge upgrade (with an awesome contract) over one year of turnstyle Wideman.

Maybe they need to make a block buster to get to that next step.

I'd do Strome and Hamonic for Wideman and Bennett.