Could you point us to a technical reference / documentation / talk that explains how spark works? it's notoriously hard to find a full technical breakdown.

Did you find this page already? I think this is quite a thorough explainer, together with the technical definitions page

TLDR - Spark

Yes I know this site, but I'd like to understand how exactly this tree thing works, what the transactions look like, and how they solved mercury's problem of splitting and recommending the coin... is there any technical paper?

Can you link Mercury's problem for me? Apart from the mentioned docs I don't think there's much educational material out there yet. As for transaction construction, I think the code is the best place to look. Either in the Breez SDK in Rust or the Spark sdk in javascript. I'm happy to link the code for you if you're looking for something specific.