> Is there a tool that makes it easy to buy Kindle books and strip them of their DRM

Yes, the most common one is Calibre, which is also great for converting formats and bulk operations. There is a "DeDRM Plugin" for it on GitHub that does what you're asking.

> Is the ReMarkable capable of running an open source OS behind the hood?

Yes, and there has been some success with this including Parabola-rM. However, there seems to be no open driver for the radio chipset (so no WiFi!).