As far as I can tell, "3-8 billion killed fish" figure indeed refers to caught fishes, not to farmed fishes (following the links it comes about from the classic 3 trillion / year estimate; http://fishcount.org.uk/published/std/fishcountstudy.pdf). I think that this part of the article is misleading; the relevant number would be the total count of fishes living in farms at any given time, as well as the number of farm slaughters.
Kelsey's messages are written in a style of informality that strongly suggests a casual conversation with a friend, and not a formal interview with a journalist. The emoji reactions have a similar effect, and there isn't an introductory message along the lines of "would you be happy to talk to vox". This overall seemed somewhat manipulative to me.
The publishing arm was sold to Wiley for £572 million, so it probably will be difficult to buy it back. I would guess that the deal would have included something like a transfer of a trademark, which would make it difficult to restart publishing afresh under the name Blackwell's.
Also, parts of their logical induction paper were published/presented at TARK-2017, which is a reasonable fit for the paper, and a respectable though not a top conference.
I don't think that's right. Half the fishes sold to humans for consumption are farmed, but if I understand correctly, a lot of caught fishes are used as feed in farms. Those fishes used for feed will also be relatively smaller and therefore more numerous. So I expect it to indeed be an order of magnitude error.