This old issue has become relevant again today - as we found out, mods from different creators are incompatible because of doubled nodes which Anno expects to be unique.

The problem is adding the same unique node with different Child Nodes from two mods. In our case <BuildingReplacements> is non-existent in the original assets and both mods do the following ModOp:

<ModOp Type = "add" GUID = '167' Path = "/Values/ConstructionAi">
    <BuildingReplacements>
        <Item />
        <Item />
    </BuildingReplacements>
</ModOp>

Which in the end results in:

<ConstructionAi>
    <BuildingReplacements>
        <Item />
        <Item />
    </BuildingReplacements>
    <BuildingReplacements>
        <Item />
        <Item />
    </BuildingReplacements>
</ConstructionAi>

Anno always uses the last one, which means only BuildingReplacements from the mod that was loaded last will be applied ingame, which can result in serious trouble with the Ai.
There is no temporary workaround for this except for an external mod which just adds this node, which is kinda iffy and not very user-friendly (i.e. people could deactivate it because they do not know what this mod does).

What I would prefer for full manual control is having a CheckPath seperate from Path. If CheckPath returns null, the content of the ModOp gets added in the specified Path. That way, all mods adding BuildingReplacements could use this structure:

<ModOp Type = "addIfNotThere" GUID = '167' CheckPath = "/Values/ConstructionAi/BuildingReplacements" Path = "/Values/ConstructionAi">
    <BuildingReplacements />
</ModOp>

<ModOp Type = "add" GUID = '167' Path = "/Values/ConstructionAi/BuildingReplacements">
     <Item />
     <Item />
</ModOp>

Optional: We could have an Argument to negate CheckPath Results to not add stuff if CheckPath returns zero.

Yea, I am currently looking into adding this.
The extra path to check was also something I was thinking about, because otherwise trying to determine that just by the child nodes would be buggy mess I fear.

As for having certain setup steps separate, I am currently also investigating a mod description format where you can specify dependencies, that will also ensure a certain load order without having to rely on the name with the alphabetical loading like it is right now (that should have been something from the start, but oh well)

https://github.com/taubenangriff/Modinfo

I am planning to include ModDependencies in the modinfo files that will be added in the next spice it up update, in case you want to use those.

ah, this request is similiar to mine, while my suggested "merge_add" is superior, if it is possible to implement:
#65