1- There is just one hypothetical threat that full nodes can ever detect: illegal inflation of bitcoins. There is no other threat to bitcoin that full-nodes are capable to detect not mentioning their inability to mitigate.
Wrong. There are numerous consensus rules that full nodes perform checks to ensure each blocks conforms to. Please refer to this
wiki article for a list. If ANY of those rules are breached, the node will reject the block.
Really? Bitcoin wiki? Interesting!
FYI: Besides ordinary transaction processing, bitcoin consensus rules serve just two canonical purposes:1) mitigating the double-spending problem and 2) regulating the rate of bitcoin issuance. You can look at the two as one big target: preserving the scarcity of bitcoin as planned by the protocol.