There is usually some energy cost associated to keeping resistance unless critically needed. Or limited place on genome.

Any organism from virus to human will happily shed over time (generations) any resistance we now consider crucial to survival, just change its environment enough.

You send them with algae and photosynthetic bacteria, which are also pretty robust. Algae is (are?)a mazing. I would not at all be surprised if you sent tardigrades to enough remote planets, that they would find some local equivalent of algae that is mostly digestable. And photosynthetic bacteria were some of the earliest forms of life on earth, likely surviving adverse conditions (I think it's still an open question whether the earliest organisms with metabolism used geochemical energy or photosynthetic energy)

it's unlikely, as they showed up evolutionarily after preceding organisms and were wholly consistent with evolution of preceding organisms. They are remarkably similar to other organisms, like isopods.

Maybe they are.