as someone who regularly use ych.comm, i can say that a seller is able to place bids under their own name, which is generally seen as them having received a bid off-site. luniwko doesn't do that, which backs up your claims of them selling three versions of one design.
if youre able to find more concrete proof of them scamming and stealing art, i'd recommend sending ych.comm an email, or reporting the listings individually!! while i don't have personal experiences of getting scammed, my gf has, and the mods did enter the chat with them and told them that the seller had sold stolen designs, though they weren't sure if the design my gf bought was stolen, but still suggested we did a chargeback! i've also seen listings who got marked with potential art theft being at least hidden from the search results, so whoever bought the designs on ych.comm should get a message about the situation too!!