So until you can defeat it, the best way to deal with the behemoth is shoot down any flyers until it you get sufficient disruption points. Is that everything? Does parking an armed aircraft at the Behemoth destination (a haven flashing pink) do anything? It could just be my imagination but it seems like parking an aircraft at the target haven stops the behemoth progress just short of the haven. Although the behemoth can still send out flyers of course. If true: feature or bug?
Probably. Dealing with infested haven is way more stressful. 
No. Or at least I’m not aware of such situation. Maybe bug… or feature. Some more experienced Festering Skies players would need to confirm.