I need to ask you this question. But need to tell you I am not an engineer. But only a legal scholar.
About %51 attack. If you have hashrate higher then %50 then you can manipulate some part of the blocks. However against old days full nodes (economicly good ones) in general dont do mining. So mining and nodes are seperate entities. If a miner with double spent or change of the old accounts or prints money nodes doesn't accept it. So the hashrate produces a fork yet normal good ones continues the good one. Is there anything wrong in this?
Can we put BIP for full nodes to check if the block comes from a %50+hashpower producer and if so if there is anything wrong they reject it. Is it possible or is it being done?
First of all, I want to say that 51% attack is very stupid, probably one of the dumbest thing that someone can do in life. You need to spend billions of dollars in building mining datacenters, buying mining equipments, setting up miners, hiring staff to take care of things and so on. If you spend so many billions of dollars (not talking about you OP), then what's the point of making 51% attack? To destroy yourself and destroy other things? Even if you attack, it can easily be fixed soon.
To sum up, I'd say that 51% attack will never happen. Other than that, some people above me already answered those questions.