Can you explain the testing process post initial QA? Specifically for playability. Do the QA testers play Runescape or are they required to be familiar with gameplay?
Core playtesting for the purpose of playability falls on everyone, not just QA. There aren't hard requirements to be familiar with Runescape but it is very much encouraged as it helps A LOT. That being said, we have a lot of RS players that vary from casual to more hardcore, and specialise in their own forms of content i.e. "PvMers" or "Questers."
Designers, developers, artists, and QA are all part of ensuring playability meets the standard they're aiming for, and each project and team might handle that differently. At a baseline, we do have at least one QA playtest for every project but also have things called bug bashes where we'll try to "QA" an update as a team.
As part of the agile workflow we also have regular sprint reviews where other teams and stakeholders are able to play and review updates, for example. There's other opportunities for playtesting across the lifecycle of a project too i won't bore you with, but it can take many forms.