There is no single mempool
Nodes can receive transactions in different orders, set different size limits or reject entries under their policy. A transaction missing from one mempool may exist in another; acceptance is not a reservation of block space.
RBF replaces a conflict
A replacement spends at least one of the same inputs. Bitcoin Core policy checks conflicts, fee increase, feerate and resource limits. The original may still be mined by a miner that saw or chose it, so unconfirmed recipients must not treat RBF labels as a guarantee.
CPFP raises package economics
A recipient can spend a parent output with a sufficiently high-fee child. Miners evaluating the ancestor package may include both when their combined feerate is attractive. It requires a spendable output and consumes more block space.
Wallets need a plan
Choose a reasonable initial feerate, preserve change that can be bumped, and understand the wallet's replacement support. Fee estimators use recent history rather than a timetable; neither RBF nor CPFP guarantees a particular confirmation time.