You will need a list of what monuments to photograph
This list should come from a somewhat authoritative source, like the government, some museum... not made up by the wikimedians or that random youtuber.
In fact, I would say that many lists used by WLM organizers are not 1:1 mappings to the "official lists": they may join multiple lists, remove monuments that cannot be legally photographed...
The local organizers may use a different criteria for local winners than for photos going to the International Contest, and that is fine. A typical case is related to sponsors some specific prize (e.g. suppose Nikon had a branch in St. Eustatius that provides a prize to the best photo with one of their cameras). You may also have rules about not giving more than one prize to the same person, etc. It is not uncommon that not all monuments participate in both local and international competition (e.g. you have two lists for the local competition, but only monuments in one of them is used for the International contest).
You need to define, and transmit clearly what your rules will be, but other than that, that is really no issue. You still have enough time to write it down before the competition begins.
The international jury will generally not interfere with your decisions of what should be forwarded and whatnot. I guess they might if a local team had a crazy proposal, but people there is usually reasonable :)
And of course, the list itself of monuments to photograph should be public. Ideally you would talk with St Eustatuis government (?) to get permission to use and publish that unofficial and unpublished so far list for your competition.