Tamur,
Nice job on the diplomacy guide. It is a great read. I have one other strategy and a response on the Map Information option.
I found that the factions I was close to and neutral with (Germans, Britons, and Spanish) would buy the map information. I offered it and let them counter-offer with an amount. In normal/normal, the offers were from 2000-3100 denarii. These 3 factions really pumped up my coffers and let me storm the Gauls.

Also I was able to negotiate Trade Agreements without any prior contact (no Map Information first). Again this was on normal/normal.