I would like to speak on your above mentioned removal of O2 from the pores.
Wood is by its design permeable to oxygen so your efforts would be better
spent persueing something else. You will not eliminate this oxygen exchange
from a wooden barrel. Hence the "angel's share" and you cannot prevent that
either there is an exchange of liquid and gasses. Just keep it as minimal as
Contributions of prior barrel contents to a beer are as you say highly variable.
You may well add a blend to the beer to get it to where you desire. That said,
I believe (IMO) that the huge size of a 50 gallon barrel and the residual that it contains
or a hogshead size vessel, lend to a stronger push on those residual flavors
than a smaller vessel say of 50 liters. Also when you introduce a culture to
your vessel, it will become a permanant denzion of that wood and take up residence
therein, Along with the residents that are already living there from the previous
contents and yes there will be some.