Just from my experience - when u have palisade wall 1-2 tile away from village idol, sometimes porting characters will spawn behind that palisade, even if they have place near idol. And your minehole is surrounded by palisade... not even 1 tile free space. Imo he just came out and spawned behind palisade.
No it has been fixed now so that doesn't happen to prevent people building mine holes on the east side of a village, going down the ladder, coming back up and being the other side of the wall. If this was after the fix then that can not or rather should not be able to happen.
The ram would seem the most likely option, if you were offline for a day then it would be possible to miss it. Normally someone ramming a palisade would rebuild it with their own gate there, but he might have rammed the other side and just rebuilt the palisade to be tidy, kinda weird though but if he didn't want to get in again then why not... alternatively he might have bashed the pally if he had enough strength, from the look of the tunnel he built the guy has enough patience to accumulate four hundred and whatever strength you need to do that.