Well, there are a few less entities, so it would be marginally faster, but in practice, nothing you would notice.

Won't help you now that you got it working, but I should probably put this into a blog article for future users.

RCV20 is just a macro with a single line, RCVDATA, in it. RCVDATA is a variable holding data sent to CC3+ via intercom. I guess you can populate this variable manually and then call the macro, but it is really a special macro made to handle intercom requests that are CC3+ commands.

The available documentation is the .pdf that comes with the XP toolkit. It is old and incomplete, but that's what is.

As for using C# from an XP, that is actually possible. @saunderl did once write a tutorial about that, but the site is no longer available and the wayback machine don't seem to have captured the actual tutorials. I've been considering looking into that myself, when time permits, but it is fairly low down on my priority list.

You don't have to make symbol catalogs actually. CC3+ is capable of just opening a folder of .png files and displaying it as it was a symbol catalog.

Obviously, you lose all the features symbols catalogs give you, such as groups, random selections, forced sheets, and so on, but for a dynamic collection, the compromise might be acceptable, especially if they have a usable folder structure in this collection.

I do hope they only add items and don't modify/remove existing ones though, as that would wreck havoc with existing maps since CC3+ refers to the image on disk.

Making symbol catalogs for such a collection could actually be a nice community project. The symbols themselves would have to be downloaded from that site by the individual user, but common symbol catalogs wouldn't be a problem if someone could coordinate that and get updated ones out there as soon as the collection changes.

Use layers. They are a great way to hide/show features. You can for example make one layer for each room, and then either have an entity on that layer that covers up the room, so hide the layer to show the room, or place everything in that room on that layer, so when you show the layer, the room appears.

Here is another blog article from me that may be relevant