To me that’s the way more complicated way to do it. There’s already a building for training units of your alliances, it’s the embassy. The building already does the function you want, so why create a new one and introduce more problems with the build grid?
It’s also far more intuitive. In real life, embassies are built in the home country and the country you want to establish relations with. This is how what I’m proposing would work.
Trade posts currently have no overlap in function between the different types. Embassies would actually have consistency between functions.
It’s quite a simple setup:
-
Embassy built on a settlement - Can train only that tribe’s units and research techs
-
Embassies built off a settlement - Can train all allied tribe’s units (and outlaws for natives) but no techs
-
Build limit off a settlement is 1 per allied tribe. Natives could have +1 to their limit to allow access to outlaws.
As for your other issues:
-
Not fitting the socket should be an easy fix
-
There’s no reason this would change how trade monopolies work
-
Wagons can just build both buildings. The trade post wagons for USA and Ethiopians can already be used to build a variety of different buildings to make them less situational
-
It’s less confusing because that is the actual function of an embassy and it’s less of a dual function than trade posts currently are