Transition: pairs/shotguns + documentation
I think it is a common if not fundamental problem we have to address when we talk about change of individuals, particularly in networks. Because there is usually less structure to connect to, everything is even more focused on (almost inevitably) one face.
If that face changes then all hell can break lose on both sides.
I am not sure how much my experience from a different background can apply effectively but here’s what you may want to consider:
Do whatever you do in pairs. Remember that any coordination you have to do between each other serves as an effective blueprint for what is necessary to run the show if one person needs temporary/permanent boost/replacement. Try to find something that is not perfect but good enough and does not get in the way (e.g. just highlight the most important 10-20 emails). If that is too expensive or cumbersome consider finding a designated “shotgun rider” or from an area of the network that is of strategic importance. Perhaps even two heavy contributors can share a “shotgun” who will try to stay up to date and help coordinate things as liaison person as well. This person need not be present all the time, but should always be invited to join important discussions to see what is necessary, and also where there are margins of error. In hierarchical environments such a “crownprince” or “suitcase bearer” is very common. Of course it aggravates typical hierarchy problems with problems of self-selection. If you want to safeguard against that you could have the “shotgun” be on the lookout for potential replacements by default and stay in the position he/she is in - only switching to giving some advice in the critical parts of business and letting the new person flesh out the rest. Meetings would still have an already known and trusted face next the the new person which will help a lot with trust and filling in the blanks.