Cold Cliff Castle
With regards to flanking
1) Does engaging in flanking combat require the combat action of all of the attacking squads
2) I figure the answer to this is no but might as well ask. Can range squads participate in flanking maneuvers?
1) Yes it does.
2) Yes only when attacking (not defending) but they must keep on the same 3 attribute facets as the attack that is being joined.
Advanced rule allows any 1 or 2 of the attributes in question may be added to the fight (only those that overlap, the remaining non-overlapping attributes are skipped for that ranged attack).
With regards to mind control?
1) How exactly does it work if I mind control a minion in one squad? Does it get ejected into its own squad and then immediately fight its old squad? (which I guess makes the Insidious One exceptionally powerful)
2) How does it work with excavating? If I mind control a creature, does it abandon the excavation. If not, can I then continue it on my turn and potentially place the tile.
1) The mind controlled minion is considered yours for that fight round from within (it must fight as is adjacent) and is added to the fight against that squad (given you have open squad control space).
2) You may continue the excavation that has been started.