1.) Hmmm... wait a second.
<browses export_descr_buildings.txt once again>
Ah, here we are:
Code:
stables requires factions { northern_european, middle_eastern, eastern_european, greek, southern_european, slave, } and building_present_min_level castle_barracks mustering_hall or building_present_min_level barracks town_guard
See the prerequisites for the stables?
Code:
knights_stables requires factions { northern_european, middle_eastern, eastern_european, greek, southern_european, slave, } and building_present_min_level castle_barracks garrison_quarters or building_present_min_level barracks city_watch
And see how the knights_stables need also the next level of the barracks?
Shouldn't that suffice?
The barracks themselves don't have any prerequisites including other buildings; perhaps I should include one settlement level for every two barracks levels as one...
I'll look out for how my changes result in the availability of the better unit types during the course of the game. Thanks for pointing that out.
2.) The list of factions is full - therefore technically for the game rebels will always stay rebels. The possibilities of changes I'm looking for concern trade buildings and unit recruitment buildings; if possible even religious buildings (as I mentioned, I wouldn't like to see catholic churches built by rebels in the middle of islamic lands).
I don't know anything about how to influence the unit recruitment of the rebels; the availabilities of buildings is just one aspect among many, I suppose.
The availability of stables, barracks and the like might influence the variety of rebel units, however.
3.) Hmmm... do you think the ai will build markets and ports more often with the fixed income boni than with trade fleets/trade base income boni?
The ai will have to construct these buildings first before making any money, anyway.
Perhaps even the character traits of a faction ("balanced", "religious" etc.) influence inhowfar the ai distinguishes between coastal and inland provinces.
Maybe we could try giving all factions the "trader" trait...
[Edit:]Or maybe the "sailor" trait... See this list...
If the ai is too stupid to realise that working towards an income it can live off is a good thing, whichever character it has, then let's push it towards doing so. 
Thanks again for pointing that out; the ai's distinction between coastal and inland provinces is something I haven't thought of.
To me making all provinces worth the same just doesn't feel right...
I'll fathom all options available that might allow making some provinces more valuable than others.
If my changes should collide with what Torn has in mind, I can still post them as user's contributions optional for the players. But first I have to make sure they work at all, of course.
All in all, different people exploring different concepts at the same time can't be a bad thing for the sake of winning insight. 
That's for what we talk about these things on this forum, after all.
-Der Ketzerfreund.