Can only molecules that are entering the cell from the outside diffuse via passive transport

No, there are many other methods except for passive transport by which molecules can move inside a cell. Some of these are:

Active transport: Active transport is the movement of molecules from a lower concentration to a higher concentration.

Facilitated diffusion: Facilitated diffusion can be described as the movement of molecule via proteins like carrier proteins.  The movement of facilitated diffusion is passive mostly, but sometimes it can be active too.

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added