Not necessarily. You don’t actually need the fluid to be perfectly sealed out, just slowed down a lot. This means that you could run it open but with very close tolerances and there would be almost no leakage. You just need to make the gap small enough for the leakage to be trivial.

As for magnetic alignment, that is all about maintaining smooth operation without losing efficiency to friction. Instead of a guide with friction you could use magnetic attraction to keep things aligned.

Yeah, but there are many good options. Magnetic alignment can keep things from touching most of the time, maintaining very good movement without friction. Graphite is a great lubricant and works even in very cold environments, not to mention it will not be all that cold given the heat passing through the system. Redundancy is also a big part of the design, making failures much less impactful. And using sterling engines for the highest draw part of the lifetime of a probe with peltier style generators there for later would allow a failover to a solid state system at lower efficiency.