Banished - Old growth vs new growth Trees

Solution 1:

"Old Growth" versus "New Growth" is not an actual mechanic. The only thing that matters for gatherers and herbalists are tree density (since this determines yields / spawnings of the smaller plants).

While I'm not certain on the absolute value of not having a cutting forester overlap a herbalist / gatherer, you definitely want to set up gathering spots in areas that have been planted / tended to by the forester, because forester's create a much denser copse than can be found on the initial map.

I'd wager your last paragraph has the right of it: a planting (but not cutting) forester + gatherer maximizes the gathering, while an active forester + gatherer maximizes total resources / map area.