To your last question about are you overthinking this; perhaps, but I'm probably going to over answer it so who am I to say.
We address this question in our organization by leveraging Section 4.3, the authority role fillers have in energizing their roles, and the responsibility each circle member has in achieving the circle purpose.
Essentially, the approach we take is that part of energizing a role is that the role fillers look for ways to cover the work of their roles in their absence. This usually involves identifying the key roles that need coverage and reaching out to circle members for help. Everyone takes vacations so everyone has this need from time to time and the everyone values achieving the circle purpose. It's also been the case that it's usually just a subset of roles that really need the coverage.
The role filler then communicates via Tactical or usually via email that "John will filling in for me as <role x> and Mary for <role y> while I'm gone." Those individuals are fully empowered to take individual actions in those roles per Section 4.3. For roles where assigning a backup was missed, Section 4.3 still applies.
For longer term absences like maternity leave I, as Lead Link, will usually assign the person that was identified to the role and then change it back later. It also usually ends up being a little bigger discussion because the commitment is longer. It may involve some of us taking turns during the absence. Whatever solves the issue with the least amount of effort.
We don't have a policy. For one, it feels a little heavy handed and restrictive. Secondly, our nimble and collaborative approach to this issue better reinforces our organizational values and has resolved the tensions to date.