Thank you for coming here and adressing these concerns.
I don't see how it would be possible to prove either of the allegations tho.
However I don't see exactly how a DDos attack would enable them to make more winning trades vs the other users...
I agree completely, Basefex could just be making this whole thing up because they don't want to pay this guy a few btc. Its not even a large amount of money, probably best for their exchange to just pay this guy off so he goes quiet.