Although I'd still like to get a good 2C, I still wouldn't be opposed to getting Marner. You talking about him improving our puck movement on the powerplay alone is very appealing; never mind everything else. Is Marner good with zone entries with the puck? If so, yeah, he'd improve our powerplay dramatically.
Back to a 2C, I think Horvat and Barzal being made available in a trade this summer is a long shot, and even more so to a team in the same division as the Islanders (us). Even if they do rebuild, I doubt they'd trade one of them to us, but maybe they decide it doesn't matter because they won't be competing against us for a long time. So, without those two Isles centers, that only leaves Benett (free agency), and MAYBE EP40 via a trade (posters here have also voiced concern about his play).
2C options this summer don't look good with only Benett and maybe EP40. Unless there's a surprise 2C that will be available via trade that isn't on people's radars.
If we don't sign Marner though, we almost have to sign Ehlers IMO. One or the other would be good signings. Ehlers gives us more cap space though.
Meanwhile, on the blue line, hopefully Morrow is ready to be on the 1st pairing with Slavin starting next season. Not sure what the front office will do about Burns though; they might extend him 1 more year.