Shemeska
Adventurer
I believe that this has been happening since before WotC was purchased by Hasbro. Also, from what I recall being mentioned by people here on ENWorld, Hasbro is very much a "hands off" style of owner unless things pear-shaped.
From what I was told by people there at the time, the failure (and expenditures) of Gleemax (and the outsourced DDI) actually got Hasbro's attention, and it was not happy. Not a clue how things have been since then.
How often would they hire D&D developers via an open posting as opposed to directly contacting a freelancer that they have experience with and whose work they like? (Not trying to be patronizing or anything. I honestly don't know and would be curious to find out how it is done.)
Not a clue. I've occasionally heard from career freelancers about interviews, and once about an open development position call, and once Andy Collins pitching an editor position that would report to him. But I don't freelance fulltime, so I don't much pay attention to how the whole thing works. Others who have actually been through the process might know more about how the positions tend to be filled (I suspect a lot of internal transfers up however probably muck with an outside perception of drastic losses, but maybe not. No way to know without a copy of the company directory a year ago versus now, etc).