As for 16 year olds--Logan Morrison, Michael Renwick and possibly Davis Young are likely to be signed, and maybe Frank Jenkins. No idea on Costantini--but most likely he's in Jr. B next year so he can play regularly.
The possible issue is that some of the 16-year olds may not want to report to Hamilton if they don't feel it as a good market to play in (even despite winning a title), preferring the NCAA route--such as former Bulldogs pick Noah Lugli, who recently went to a RBC Cup with the Wellington Dukes.
I know D'Agostino and Van Loon will be on the roster, but who else?
But yes, there are many holes to fill on this roster, which will require the Bulldogs to pay up on some of their picks to prevent them from committing to NCAA schools and choosing Jr. A or B over playing in the OHL.
To your second point, I'm skeptical of that being an issue. We have yet to have a single player refuse to report for Hamilton save one import draft player from my understanding.
Foremost, as you've said, we just won the title and will (hopefully) keep the same coaching and management staff intact. There is something to be said about a team who goes from the basement to a champion in the span of three years, and thus far under Gruden, hasn't yet failed to make the playoffs.
Second, Hamilton is the 3rd largest metropolitan area in the OHL (after the GTA and Ottawa). That means, any player from the city proper, it's amalgamated communities, or Brantford, Burlington, Grimsby and Caledonia aren't very likely going to refuse to report to the team down the road, both out of pride but also being able to live at home.
Third, apart from Mississauga, Guelph, Hamilton has the least travel average travel distance to the other OHL markets, and apart from them and Oshawa, is the closest market to the massive GTA. That's a huge benefit for the franchise, and even more so since you are also absurdly close to Niagara, Tri-City and reasonably close to London as well, who I am pretty sure make up the 4th, 5th and 6th largest other metros.
Last, one big reason why a player will choose to go to Jr. A or B over the OHL is ice time. A borderline player in the OHL isn't going to see the same amount of ice time on a Jr. A or B team. Generally players who have higher aspirations understand that not getting ice time cam be much more detrimental to their development then playing at a lower level. That said, when an OHL team is in a rebuilding phase, usually ice time gets spread out quite a bit more and suddenly the level of competition for spots isn't as high making it a more appealing prospect.