Hi
Are you sure you really want it backed by a hexagonal graph? What is the reason for this if you want to use navmeshes for dense areas anyway?
In the meantime, as for 1. Take a look at https://arongranberg.com/astar/docs/graphupdates.html in particular this section: https://arongranberg.com/astar/docs/graphupdates.html#direct.
2 Pathfinding is done per node, so if you want different information in each triangle, then each triangle has to be a node.
3 Generally it is better for agent-agent avoidance. agent-obstacle local avoidance is not that well optimized at the moment either.
4 Yes, you can use multiple graphs and switch between them by sending a different graphMask to the Seeker.
This requires some minor changes to the movement scripts just to pass a new parameter.
See AI of different size in Recast Graph
5 Yes, that is also covered by the links above. The cost depends on what you need to change, changing the penalty of some nodes is very fast, however changing other things such as walkability can be slower because of other data structures that need to be recalculated.