Finding such a developer is a big challenge. Probably nearly impossible. Any good MediaWiki dev seems to already have a real job or is hired by the Wikimedia movement already.1) I agree with Gergo's point quoted above, and within the context of the current proposal, would recommend budgeting for at least a 20 hour/week developer supporting Dan with code review and integration, ideally someone with prior MW experience. This cannot entirely eliminate integration effort on the WMF side of things, but will ensure that the people who have the greatest interest in seeing the project through to completion are set up for success in doing so. With or without this, let's really carefully negotiate what exactly everyone's commitments are so we avoid a repeat of phase 1.
No, it doesn't need to exist with MediaWiki. We wanted to create a tool that ran on a production grade platform with production grade support. The only organization offering that is the Wikimedia Foundation. The only production grade platform operated by the WMF suitable for this application is based on MediaWiki. That's the reason why it was build as a MediaWiki extention. What other production grade platform could we use?2) I say "within the context of the current proposal", because I would ask you to give serious consideration to the following question: Does GLAMWikiToolset need to existing within MediaWiki? When it was first developed, we didn't have OAuth (so a tool outside MediaWiki couldn't perform user actions), and our APIs were less mature. Today we have many examples of external tools that are doing amazing things. Magnus' tools have made tens of millions of edits to Wikidata. The Wiki Edu Foundation has created wizard.wikiedu.org and dashboard.wikiedu.org for managing student assignments and courses.