What is the default relaying policy for RBF double spend transactions?
Nodes will only relay if mempoolreplacement hasn't set to false (most nodes probably do relay because the default is true).
There are serval rules for accepting a replacement after BIP125 (check the BIP for more details). In general: The replacement must pay greater fees than the transactions it replaces - if we did the bandwidth used by those conflicting transactions would not be paid for.