Hold on real quick, wait, hear me out.

All that needs to be sanded away is the protective floor coating for you to proceed. In my experience, it's a kind of shiny polyurethane coating. You will know you're at the base/raw wood's level when there is no more shine and it "feels" like wood again rather than it would be the usual physical feeling of a wood floor, like it feels glossy. I hope that can make sense.

That's all you really need to sand away. I wouldn't worry about the paint. Once you have that protective coating off and the raw wood is unearthed and it feels smooth you can go ahead and stain it. The stain will take away all those paint stains etc.

In any case, staining the floor with wood stainer is probably something I'd imagine you'd planned to proceed with anyways ? If you are unsure you can do a little test area to see if the wood takes the stain. No harm if it doesn't because you can just wipe it away.

Source: My uncle is a wood floor guy and we did some floors together at one point.