I am currently working through the same thing as you are. For something as important and central to the world of Ravnica, the Guildpact isn't very well described. Fortunately, it seems that the story team over at WoTC is consistent in their desciptions of the Guildpact's power.
Based on various flavor texts, original Ravnica: City of Guilds, Dissention, and Guildpact novels, and more recent stories I gathered that:
- The Guildpact is a very powerful law magic
- It mainly deals in establishing what are each guilds' domains and roles in the city-plane
- Its power is very subtle and can be felt through:
- changes in "luck" (see Festival of the Guildpact flavor text)
- enhanced successes of those who are fighting against those who would infringe on a Guildpact law
- possibly disastrous attempts of subverting or infringing on a guild's domain
- Some undead spirits work to uphold the guildpact and are directly powered by it (Guardian of the Guildpact)
- Both the guild members and the guildless individually are not under protection or forces of the Guildpact
- The guild members gain non-Guildpact related benefits just for being members, and the guildpact law protects their endeavors as long as they are within a guild domain's parameters
- Guildless can only expect the Guildpact laws to push back on any endeavors they might have that fall under one of the guild's domains (like starting a bank or a medical clinic)
- Anything that's personal and outside any guild's domain is not covered by the Guildpact at all (Vinditctive Vampire)
The guildless are often on the losing side of the deal, and are prone to attempts against it. There are many clues for this, but my favorite is from the Enemy of the Guildpact card.
Dimir secrecy has been broken since the arrest of Shadek around 10,000 ZC. The new guildpact includes their existence.
There aren't exceptions for arresting anyone. Rather, the arrest in question contradicted a rule in the Guildpact law and caused it to collapse on itself and triggered a fail-safe which resulted in making Jace Beleren the Living Guildpact. He now alone has the power to change the guildpact laws and as a living, sentient being should theoretically prevent it from collapsing to further paradoxes (or create more loopholes). Unfortunately, it seems that his presence on the plane of Ravnica is necessary for the law to function, and he likes to move about.
So, if someone violated a Guildpact law, they will get arrested with relative ease and have a hard time breaking out. If someone broke a non-guildpact law, even an internal guild law maybe, then things would get a bit more mundane.
D&D wise, I'd give advantage or disadvantage to players if they are directly fighting for or against the Guildpact respectively.