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?