They definitely didn't want to be outwardly poaching top guys from other teams after they entered because it only signifies they are failures and whoever it is from another team is coming in to fix it. But the guy was there from 2013.
The rumour being he left because he wasn't being listened to, however would he have stayed there since 2013 being ignored, not too likely. People who are being ignored now but weren't a couple of years ago makes me think he was more involved with the bad concept and now he's being marginalised as he was part of that team that failed.
As for Mclaren putting their foot down... there isn't much indication of that, a guy choosing to leave because the team doesn't listen to him doesn't really give any insight into what Mclaren are saying. Also Mclaren/Dennis are hugely responsible for the mess themselves, insisting on size zero(which they're moving away from for that flared air exit/under cut) which required a smaller engine and insisting it had to be 2015 to enter. Honda made a bad engine but I don't think Ferrari or Merc would have come close to the engines they have both in speed or reliability, if their development program had been cut in half and then trying to both develop new engine while constantly trying to fix the other stuff that isn't working yet. Honda were utterly moronic to agree to such a deal, but Mclaren both obviously approached Honda and gave them undeniably stupid design goals to try and achieve. Even if they'd relaxed the sizing demands it's unlikely they'd have made something genuinely good in 18 months, but considering how many failures over the past two years have been compress/mgu-h related and considering how much easier those would have been to improve and get more performance from in a larger package... I think the majority of the blame goes to Mclaren.
Very important point you raise regarding were the performance of the other manufacturers would be if they had their development programs drastically cut. They too would have suffered catastrophic failures and much slower lap times. I am sure this would have resulted in a complete regulation change to testing and development but because it's only one manufacturer and one team attempting to develop and race under these impossible regulations, it's ignored. Honda could walk away and yes I know they have left F1 before but the landscape of F1 has changed. Manufacturers are not lining up to get into F1, however there seems to be a line forming to enter Formula E.