Another point I think I left out in my essay post, there is a bug where if the AI attempts a forced invasion (Naval at least so 'forced' is more a guess) and a standard invasion from and to the same regions all invasions are cancelled. This is why if you look at the ai_db in Medimod you will see a strange entry...
<decision_entry>
<!--
cancelation of invasions override
-->
<min_entry is_neighbour="true" borders_all_our_regions="true"/>
<max_entry is_neighbour="true" borders_all_our_regions="true" num_settlements="3"/>
<faction_attitude can_force_invade="false" continue="true"/>
</decision_entry>
I have set a max number of settlements because it almost never happens with larger factions. Strangely it also seems not to happen if they have multiple settlement blocks (borders_all_our_regions.)

This also brings up another interesting phenomenon (aka bug) wherein somehow it is posible on occasion for borders_all_our_regions to be true when the two in question are not even neighbors. Thus my use of both.