All of the OP suggested alternatives are modern. Back pre-1400 war was pretty much it as far as diplomacy is concerned. You couldn't even reliably sanction someone because of smuggling. (today, of course, you can't rerliably sanction someone because of Russia)
- Peaceful protests of course happened, but they aren't documented very well.. because they were so common. Paper is cheap, as well as papyrus, wax tables and clay tablets.
- Trade sanctions: controlling trade always has been a tool of power. Who gets the tolls, the staple rights, the mill, etc. ? A step further towards war is blockading ports or trade routes.
- Sabotage was less common, because there was less infrastructure and destroying something required an army anyway due to the lack of explosives. Raiding and plundering, burning fields as a war of attrition instead of a confrontation was common practice, however.
- Propaganda: not as flexible as the modern version, but sponsoring religions/missionaries is an ancient practice. There still is pretty direct stuff, eg. the Mesopotamian kings that deified themselves, or the Egyptian Pharaohs that put up steles to commemorate their victories.
- Outright bribing was less common in times with less monetary means, but promising a count a promotion to duke if he switched allegiance from one king to another would have been considered.
1) No, they really were NOT that common. At least not on a level that was taken seriously. There is a very good reason for this. FEUDALISM! The Great Chain of Being, Castes. All that. People were programed to think they were born into a position and HAD to listen to those above them for the most part. Education was not common, what was there reinforced the Great Chain of Being. You know what getting excommunicated in pre-1400's Europe meant? It was a death sentence. The Great Chain of Being was a part of the Catholic doctrine, and thus to deny the great chain (aka: protesting your better's actions in any way shape or form) was heresy.
2)Actually only in DF, period wise it was still not a valid method because most places were built to be self sufficent. Do you know how HARD it was to maintain communications and other things after the fall of the roman empire? Once their roads broke down? Trade in europe almost came to a stand still until the Bergs began to form (proto-cities). I'm not saying it wasn't happening, but the general concensus was that either A) my vassals give me the stuff I need from taxes and may request a tiny bit from me if they need it, or B) I can go raid my next door neighbors at lance and sword point and FORCE them to give me what I want, or C) I can do without.
If we're talking a full blown siege or blockade? Again, not common until the age of sea except for land based sieges. And those are basically what we go through now. NO GIANT CATAPULTS! NO TREBUCHETS! Just a bunch of guys camped outside taking any supplies meant for the City/Castle/Keep until somebody starts waving the white flag.
3)Sabatog has, and always WILL be a valid method of attack for any period. Espionage and assassination are the 2nd oldest professions in the world. It might not be attacking infrastructure. But poisoning a well is almost always just as effective. Or setting a field of crops on fire at night.
4)Yeah. Missionaries were about it. And that only worked on Heathen lands, and didn't stop the Crusaders from coming in and still lopping off christian heads. Sorry, the crusades ended more christian than Muslim lives by the end of them (and that is NOT including the crusader casualty list, nor those who the Muslim's killed. That's taking the kill list of the Crusaders ALONE).
5)This is the only place you've got it right. Not much, except in terms of "Hey, you want a higher position in MY government? and a huge ass portion of the land I'm about to conquer?"
These are only pieces of the truth that weren't true by the year 1400. This understanding of the medieval ages, as I said before, is the result of learning about it from the media and public education, rather than by actual study of the history.
1) Feudalism, class systems, etc. only applied to relations between nobles and commoners. The various kings and emperors would have had to communicate by diplomat. The Capetians unified France by diplomacy as much as by warfare.
2) It is ridiculous to think that breakdown in communications remained that serious 1000 years after the fall of Rome. There was, for example, a popular historian from Belgium who theorized that the reason for the decline of Europe was because of the breakdown of Rome and the invasions by the Islamic world. His theories have since been rejected because the data shows that trade continued at similar levels throughout Europe despite the fall of Rome and the incursions by Islam into North Africa and Spain. Furthermore, even the bergs you speak of existed way before 1400. For example, the Hanseatic League, a powerful German trading alliance of cities, began in the mid 1200s.
4) Not just missionaries, but other types of evangelistic efforts were common during the time of Rome, when Christianity was proceeding from the top down. The only reason why one sees less of it in Medieval Europe was because of the relative homogeneity of communities and the relative freedom of doctrine that existed among the illiterate. That the Europeans later sent missionaries to the New World shows that it was a strategy they too endorsed, only they lacked a subject population to exercise it on - and there would have been missionaries sent to Islamic populations (I believe there were any way) if not for Islamic regulations banning Christian and Jewish evangelism.
5) Bribes can always take many different forms. No need to limit it to land or position - gold was certainly desired, valued, and available, as seen in Beowulf. And isn't marriage the common reward in fairy tales for the successful hero?
But in any case, with regard to Dwarf Fortress, if the elves bribe your nobles, perhaps you can expose them and have the hammerer execute them legitly. For sabotage, they might lay traps in trees to specifically target civilian woodcutters. As for religion, it gives your priests something else to do. Prevent your dwarves from straying from the Way...
NO ONE EXPECTS THE DWARVEN INQUISITION
(Semi-off topic)
Regarding international systems: The Holy Roman Empire was basically an international system and would have been able to sanction its member states. The constant state of warfare between the Italian city states meant that a de facto balance of power existed between them. Furthermore, the reason why no alternate system existed to the Catholic Church was that it fulfilled the needs required of such an international system. The only reason why Europe later developed laws of warfare (post-1400) via the Treaty of Westphalia is the war that resulted in the Treaty of Westphalia was caused by events that also discredited the Catholic Church (the Reformation).
Edit: I was somewhat in anger, because history is my passion.