Here's RTW/MTWII diplomacy.
You have a hidden "relationship score" with each faction. At specific thresholds the AI will allow you do perform various deals.
Specific actions raise or lower this score. Anything "positive" boosts it, but its been years since I looked up hard numbers. Trade rights, selling map info, becoming protectorate, trading regions, and ceasefire all raise relationships. Giving money in tributes also adds relations relative to the amount of money.
Going to war, breaking agreements, allying with nations the AI is at war with, and failed negotiations (any time the AI gets mad and closes the diplo window on you) all lower relations.
Heres the problem. You also passively lose relations with every AI that has a land border with you. Its to represent territorial hostilities. This means neighbors end up in a perpetual state of war. You can "appease" neighbors to offset this, mostly by giving them tributes, but eventually they will go to war, even if they are allied.
They may go to war earlier or later based on your military strength relative to theres. They may also go to war if pushed to by Alliance politics.
So what does all this mean? It means the AI is retarded. Dont seriously expect lasting alliances, from ai, or between ais. Just expect and endless state of chaos with occasional peace treaties. Also, Protectorates. They are very difficult to acquire, and not very benefitial. You get a portion of the protectorates tax income (20%?) and have alliance and military access with them. However, the AI will still have very low relations with you, because to even get them to agree to protectorate they must be thoroughly beaten into a pulp first.
So Protectorates betray their masters at the drop of a hat, making it far better to just wipe them out and be rid of them.
As for your alliance comment, go for it. It wont really help. AIs are far more likely to break alliances than join you in war, so you might have 5 allies, but 4 of them jump ship when a random neighbor declares war.