I am fairly sure that the "Replaceable storage" and "Water-proof" are contradictory. The only water-proof e-ink ereader with removable storage that I can think of was the original Aura H2O. That had a waterproofed case that had to be opened and then resealed to get to the internal card. It also had an external slot under a sealed cover.
For the rest, I think you are looking at either a Kobo or Kindle. They are both Linux based, can have the registration avoided or done with fake address and do not need WiFi on afterwards. Both have models with buttons and are lasting well. I know older Kobo devices will run off power with dead batteries. I don't expect to test any of my recent purchases in that way for at least a couple of years, but, I would expect them to work.
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