You're doing the Product Manager's job, in my opinion. They're the ones who have to justify to senior management why something is being done, and when. The devs really should just get on and do it once it's been agreed.
Involving devs after the roadmap has been decided is too late.
I agree though with what you're saying, regardless of the technical capacity of the PM, it's not really up for him to decide the capability of the teams, or the proposed technical solution. He should be managing those with the engineering teams, and their thoughts should be reflected on the roadmap.