unfortunately, I am not sure if the p2 and p3 mercs would show up in the structure tree.

sounds like the easiest solution is to promote the village merc into town merc with ~2000xp, then promote the town merc into the city merc with another ~2000xp, and then add a modification to set xp required for promotion to 0 to the phase up technologies. you could add villageMerc, townMerc, and cityMerc classes as handles for the different modifications. you could have an infantry p1 merc promote into a cavalryman in p2 with this approach just fine.

You can get these features in ModernGUI mod by Atrik

Is this still compatible with the most recent patch?

Lag is only observable in 3v3 and 4v4 TGs.    Today I made a mod which removes the walking and attacking animation of the jav cav. I limited the animation to A23-style low arm javelin throws of the rider and completely removed the walking and idle animation of the horse. That instantly reduced a lot of GUI lag for the Carthaginians. It would seem that cavalry model and animations are a major source of GUI lag. Interestingly, camels lag less than horses because they have less attachment points.    I will update you on lag after I remove all cavalry animations.    Some experiments showed that having too many objects in your selection group can also be a source of stutters. Today I took over the base of a player who makes less barracks than me and I also experienced shorter durations of stutter when accessing barracks. Huge GUI crashes happen when I am trying to select a large group of units.  Again, strangely, in 1v1, selecting the same group doesn't crash the GUI as much. The total number of visible units on the screen are the same in 1v1 and 4v4 situations. 

For example in historical patch i have it so you can buy merc from market p1, but they are rank 1 and can't promote.... so can be strategic early on if needed, used well.  But almost useless later in game when champs cost pretty much the same metal.  wondering if it would be possible to create a rank2 version that supersedes the p1 rank1, and same thing agian in p3.  instead of having 3 different ranks to choose from, cause that could get confusing.

I've made much better than this. But no one likes it.