Li-ion swappable 18650 in parallel - preventing mixing charged&discharged cells
I would strongly recommend against such a scheme where you connect, disconnect and then reconnect cells in parallel again ! You can only connect Lithium based calls in parallel after properly balancing them. This means that they must be allowed to reach the same voltage and stay connected in parallel

Sure you can use a fuse or PTC or whatever to protect the cells charging each other but it is still not an elegant solution. I predict that the situation will occur where the user will be unable to find cells that can be inserted without the fuse activating.

A much better solution is to make packs of balanced batteries and connect/disconnect them as a whole.