Valid public key check in transaction output Let's say I build a P2MS output script using uncompressed public keys, and let's say too that these public keys are not valid, i.e. they do not represent a valid EC Point and they don't start with 0x04. 
Will this transaction be broadcasted and included in a block or, among all the validity and standardness checks, a peer also controls that public keys are valid and rejects the transaction if they aren't ?
Thank you very much !
The standardness check will verify a multisig output for valid EC points. If these points do not pass this verification, the output script will be considered non-standard and not propagated by the implementations which I know of.

This can be verified by checking the implementation source.