using and refinishing existing pine subflooring

Under a layer of particle board, I have 12" wide pine planks that I would like to sand and finish (rather than adding an additional layer of wood flooring.

I noticed that the trim around the doors as well as the baseboard is level with the surface of the particle board which will leave a space once it is removed. Has anyone had to deal with that issue? I would hate to have to move all that trim. I am considering just adding a piece of trim below the existing trim to fill in the gap.

The particle board is nailed to the pine sub-flooring about every 4". I haven't removed the particle board yet (I can see the pine planks from underneath in some areas), but I am afraid all those nails will have significantly marred the planks. What is the best way to deal with all the holes?

Is there any issue with the gaps between the planks (on average about 1/4")?

When sanding the floors, I would suspect that the nail heads in the sub-flooring will tear the sand paper. Is that an issue and what is the best way to deal with that?

Thanks for any help,
jgd