As to whether or not they've dumped their premine, given the unbelievably low Bytecoin exchange volume I simply think they haven't been able to. In order to dump a premine you have to have buyers, otherwise who are you selling it to? Leaving aside HitBTC, which is known to be untrustworthy and filled with fake trades (1 2 3), Bytecoin does about $2/day in volume. The most heavily traded day was June 21 when they did around 1.5 BTC in volume at a price of around 10 satoshi per Bytecoin. In the 245 days it has been listed on Poloniex, even if they were at peak every single day they would only have been able to dump 3 675 000 000 Bytecoins of their stash of 147 882 114 539 Bytecoins, which is 2.49%. Given that they're not at peak, the best they could have achieved over the past 245 days is a couple of hundred USD.
Not sure where those numbers came from. A lot of BCN traded on polo in the first month. Here's is the daily graph. The first scale line on the vertical axis is 20 BTC.
There were several days above that (most notably the first two days), and many more days in the 10-20 BTC range. You also have to consider the OTC trading, and the pre-poloniex trading on cryptonote.exchange.to.
My guess based on this data is a few hundred BTC of the premine was likely dumped (BTC was worth around 600 USD at the time) in the first month or two, but obviously nowhere near all of it. 300 BTC at 8 satoshi is still under 4 billion BCN. I have no idea how much was dumped in the later months, but once the volume dried up your numbers are pretty much right so no more than a few more billion.