I think that maybe your approach to realm rulership is the key to this one. Without pre-judging how successful that was, I find that a lot of hand-waving, eyebrow raising and concerned peering does as well as a heavily-systemed approach for social interactions unless you go to a system (like Burning Wheel or Dogs in the Vineyard) that takes such things really as seriously as combat.

Now - I'm not advocating ignoring social stats/skills/traits etc but rather stepping back from pure rolling-for-success to a more abstract low-system approach of taking them into consideration when your NPCs interpret what the PCs say and do.

Difficulty and Impossibility I think the core issue of most social interaction/combat systems is that they assume that everything is up for grabs. You can literally convince someone that black is white should you roll well enough.

Work out the core-tenets of your NPCs - the things they just won't be shifted on no matter what. Then work out their other goals, beliefs, alliances etc and attribute a degree of difficulty to each of those and apply this as a modify to any tests. A negative difficulty is fine as well, some times folks don't need much convincing to do something they really want to do anyway.

Consequences Under some circumstances it's reasonable to up the stakes a bit. Failing to get a treaty might not just mean no treaty - it might also mean you look weak in the eyes of your fellow nobles. A failure to convince a mercenary captain to ditch his current employer might mean that he percieves you as dishonest and makes extra efforts against you or bad-mouths you to everyone he meets.

Finally If the system isn't achieving your goals I'd advocate pushing it more into the background and highlighting roleplaying over the system.

Now - I'm not advocating ignoring social stats/skills/traits etc but rather stepping back from pure rolling-for-success to a more abstract low-system approach of taking them into consideration when your NPCs interpret what the PCs say and do.