I'm very interested in editing diplomacy in general, in particular the weights that leaders attach to technology and cities. I don't think that any trade should be "redded out", because there should always be an offer that will make them at least consider it, and there are a few other changes I'd like to implement as well. Can anyone point me in the right direction to start this? As of right now, I don't even know what files to look at.

In XML you have Civ4LeaderHeadInfos.xml to control some basic aspects of diplomacy. Very little of the Diplomacy code is exposed to python so if the XML isn't enough you'll have to dive into the SDK.

These values are kinda tricky with no documentation. Does anyone have some information on them that I could reference?

Which values? The thresholds are attitude requirements and the delays, memories and memory decays are modifiers.

So, for an example of a threshold if you want to make open borders more difficult you take:

<OpenBordersRefuseAttitudeThreshold>ATTITUDE_ANNOYED</OpenBordersRefuseAttitudeThreshold>

And change it to:

<OpenBordersRefuseAttitudeThreshold>ATTITUDE_CAUTIOUS</OpenBordersRefuseAttitudeThreshold>

The contact delay defines how often they can ask for something, increasing the delay will give you more turns between requests. Memory settings aren't too complicated, you have the decay which basically tells you how long they'll remember the action, attitude percents which affect how much the action affects their attitude (basically this number divided by 100 is the change to their attitude but it changes as it decays).

That should just about cover it. Only advice I can offer is only change one or two things at a time, some of these will have side effects that aren't readily apparent. For instance if you make open borders easier to get more civs will have a possitive modifier for open borders and you'll have fewer wars.