Well, I didn't get as much done this week as I'd hoped, but here's where things ended up.
I spent most of the (somewhat limited) spare time I had to work on projects in the last week on understanding both the architecture of the mandy prototype and ActivityPub itself. I like to write tests to check my understanding of how things work, so I got started on adding a test for the unmarshaller.
That's as far as I got this week, though. I got tripped up for a bit by the fact that SRFI-64 test output displays strings in a test-equal form without their enclosing quotes. I figured that the alist values were strings, but I didn't realize initially that the alist keys were as well. So there was a bit of "those look equal, so why is my test still failing?" going on.
But I plan on continuing to poke at the codebase, and this challenge was a good motivator to get started doing so. I'd consider that a success. :)