Alliances would be better served in game terms if you had to choose to actively support your allies or not. Like the good old space 4x games: Don't make alliances/promises you can't keep as it makes your rep suffer massively etc etc.
Allies need to come to your aid or break their alliance with you. Simple as.
Otherwise, what is the point?
I would also like to see allies requesting miltary support against other factions as you mentioned above, far more often. Likewise, there is no reason why an ally can't cry off supporting you or ask for payments to cover the cost of deploying their forces etc.
Good relations with an ally should almost never lead to a backstab. Historically, nations with common culture or mixed royal lineages etc rarely (as in full on - again in game terms a skirmish, a battle sack a city and call it time, negotiate a peace etc. IN TW its always a struggle to the end. Dull.) went to war. Hannibal after Cannae didn't march on Rome because he sincerely believed the Romans would concede defeat and negotiate peace. D'oh. But it was the way war was waged - it was too expensive to flog people into annihilation. In the middle ages, intercene wars spring up when dynastic changes occured - in game terms this would be cool. A new monarch with low influence, or bad boy traits or an ambitious regent should have profound effects on diplomatic situations. Gold, assasination or war being the only options .. What about factions offering tribute/bribes to break alliances and side with them in a conflict etc? There is huge room for expansion of the diplomatic aspect of the game I think 90% of TW players are bursting to see.
As it is, you can basically expect nothing from an ally except inevitable backstab. Makes it pointless and boring as a game feature, which begs the question: If we are all into making this more relatistic and immersive, why hasn't anyone done a proper complex rework of the diplomacy? Inclination or just technical barriers?
As I play this game more and more it becomes duller and more futile. There is nothing to it but endless conquest through conflict, even with loads new unit types etc its still the same. Played it with one faction, played it with them all. Diplomacy is the depth that keeps you coming back time and again and its just never been finished in theTW series.
I read a press release from CA recently, paraphrasing a bit but it basically went: 'We know what fans of the TW series want, and its more war! More units, bigger battles, better special effects.. ' Well, frankly, its not. As evidenced by all the mods that aim to make what is effectively a RTS battle game into something more compelling long-term. Yet they never seem to acknowledge this is what people want - presumably it isn't? Who knows.