Walls can be useful, especially in combination with "needles" (the spikes) - delaying and damaging them until they bust through, potentially killing or fainting them if they're weak enough.
As for whether or not they could "spawn inside"... I think they spawn some distance away from their target, so potentially yes, they might spawn inside (didn't test though)
Random pretty much, shouldn't be that rare though.
Raids of level 50 with flower totems I think? (refer to John's guide for detailed info, don't know exact details anymore)
Blue smoke thingy to increase the time until a raid occurs - alternatively, use leaf mascots and such to decrease the number.
If the decreased number is higher than the raid count, a raid won't occur!
I assume you're still on the 0.0.12 version of this thread, in which case, it should automatically repair itself once the raid ends.
(If not, try /repall command, it should repair everything, or wait and finish the next raid)