If it generates, at most, 57GB per day[1], assuming 22h operation (2h for transmission), the sensors are generating 2.6GB/h or about 750kB/sec which is just about 6Mb/s (unless my math is wonky.)
[1] "JWST can produce up to 57 GB each day (although that amount is dependent on what observations are scheduled)."
That is the average rate over a day. If storage is not available to buffer it, then a sensor's peak readout rate could easily exceed the transmission rate.
But also "The actual data rate depends on the number of detectors simultaneously in use, their exposure parameters, and the precise timing of when their exposure readouts arrive in the ICDH for processing" - in a reduced operation mode, they can turn down the number of sensors, etc., to keep the data rate below the live transmission rate.
Yes, but a servicing mission for replacing the memory would still be an unlikely operation to be able to do. The most likely use for the docking port is if the electronics onboard exceed their design life, but JWST is running out fuel (to maintain its position at the Lagrange point) so they basically strap a "jet pack" on to the satellite to keep in operation. This has been the business case of some companies trying to do this for GEO satellites:
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