Well, I mainly work on grade 2 domestic properties but in several hundred jobs, over decades I never had an issue, ever, with a CO with regards to sanding floors dating back to the 15th century. I would be very surprised if you had an issue.
You are correct in that grade 2 listing covers the interior as well as the exterior and also that if you are unsure you should always check with your CO. I have replaced thousands of boards, almost all of them being reclaimed.
Dozens of listed floors are sanded, often badly! with cheap, industrial hire sanders every day. I have never heard of an issue even with unsympathatic work.
I use a range of the top professional machines all of which I own and you cannot hire.
It's a question of assessing the thickness of the timber, for boards I don't use a walk around sander if they are less than 16 or 17mm. For 15mm I only use hand machines. For less than 12 to 13mm I wouldn't sand without removing and strengthening the boards or fitting a subfloor underneath.
In addition, for floors earlier than around 1820, ie Georgian, I offer the gentler hand sanding method to preserve the dimensional topography.
After a certain age I would refuse to use a walk around sander. You cannot really belt sand a 17th century or earlier floor as they are too uneven.
You have to be very careful with parquet floors as some are thin 9mm overlay, often from 1890 to 1930 and some have already been sanded close to the tongues.
A reversible finish is simply one that never cures and can be softened and removed by it's original solvent. The main ones are shellac and wax which can be removed with meths, turpentine respectively, solvent stripper or hot washing soda. Oil finishes are not reversible but are pretty weak and can often be removed with similar methods.
A diyer should be able to apply most clear finishes if they use a brush on straight grained floors, herringbone can be more difficult.
Hardwax oils are probably the easiest to use for DIYers if you follow the instructions.