We often customize objects for a scene (mat and/or geometry), so I guess I'll have to find them and move them to another dedicated layer ? If we use the library objects without modification, an xref is enough.
Yes, in this case i suggest to use a separate layer. You can use a temporary Forest object to import the models in the scene, move (and rename) them to the new layer, and once modified, assign to the production Forest objects.
What happen if we modify the material or the geometry of "merged into scene" objects, is there a chance that it will be overwritten at some point if we leave them in the forest ref layer?
Yes, depending of the option selected in "On duplicate Material", in the Library browser: "Use scene material", "Replace from Library, "Ask to user".
To avoid that, just use the first option or rename the material in the imported object.
As a wish, would it be possible to have more meaningful names instead of _forest_library_QL6D5IQ or _forest_library_OKTPHTQ. Where do those names come from ? (those are maxtree objects)
Those names are generated as an unique identifier for each item of the Library. In this way, we can check easily if the items are already imported in the scene (to avoid duplicates) and assign them.
Ufortunately there is not a simple way to change that. We cannot use directly the name of the object, because there is not guarantee that it's unique.