It sounds like you’ve got a solid plan for tackling your cabin sole restoration! Here are some tips for each of your issues:
Cracks between the wood: First, clean out the cracks thoroughly with a putty knife or small chisel to remove any debris. Once they're clean, consider using a wood filler or
epoxy designed for flooring. If the gaps are significant, you might also look into slivers of wood that match your floor for a more natural look.
Water staining + dirt: For the stains, a gentle scrub with a mixture of water and vinegar or a wood cleaner can help. If the stains persist, you might consider using a fine-grit sandpaper just on those areas, followed by a touch-up with a suitable finish to blend them in.
Slightly lifted boards: If you can’t get underneath, sanding the lifted areas flat could be a good temporary solution. Use a belt sander or orbital sander, but be cautious to not sand too much, as you don’t want to create dips. For a more permanent fix, you might want to try applying a little water to the lifted edges; sometimes, moisture can help the wood settle back into place.
Sanding the entire floor can be daunting, so focusing on these targeted areas first can save you some effort. Once you’re ready to do a full restoration, you’ll be in a better place. Good luck with the project!