MacBook – Will video and photo editing wear out a 2015 macbook pro’s SSD significantly

Encryption happens on-the-fly. If the data was written to the hard drive, then passed through an encryption algorithm that then deleted the written data and re-wrote it, encrypted, that would be hella inefficient, and would largely defeat the purpose. I think the answer to your question is no, but of course remember that if you're going from a data set that's not yet encrypted, then all of that data needs to be processed, so that initial process of reading, encrypting and re-writing data will have an effect.

No - The SSD card is a discrete component on the new retina MacBook Pro and can be ordered as an Apple service part. Time will tell if an aftermarket supplier decides to supply parts for that model.

Even though it's not soldered, it still is not a user-servicable part.

As ArsTechnica reports:

While Apple uses the latest SATA protocol, the connector is physically incompatible with either Mini PCIe or mSATA.