I ended up disputing. Got my credit today (from the dispute, not the Nats). We'll see if it holds. I was thinking of buying a partial season plan but this has left a sour taste in my mouth - there was zero communication. When I called I was told it would be "several weeks" for a refund (back before the Nats put on their website a estimate of 7~10 days).
It seems like several weeks was spot on - inexcusable. To be honest they shouldn't have charged people to begin with (talking about Single Tickets) and instead issued a hold and charged you only if they made the next round.