Ok - so I've filled the gaps and sanded back. When I did the sticky-tape test, the intergrain did come off, so I'll followed directions and sanded back to bare wood. However, there are a few indentations in the wood - either patterns in the wood or indentations from dropped items - which are still a little "glossy". I don't want to sand back too much - would it be safe to assume these smalll areas won't be an issue and I can just go over them, or is it worth trying to use some kind of stripper to get it all off?
It's one of those things where, in a perfect world, you'd want none of the previous coating left. Will you see any adverse effects if you coat over? Probably not, but it's hard to give a definitive answer. I believe even if you were to use a stripper, you'd find it difficult to get the remnants out of the crevices. You now have plenty of surface area the new coating will bind to, which should offset any tiny areas left to which it won't adhere.
If this were my floor, I'd take the chance and coat over. The amount of work it will take to get an ever depreciating amount of coating off, I feel, outweighs the benefit.
Let me mention @TedBear to get his thoughts.
Yeah that was my vibe too, but I've never used a stripper before so wasn't sure how successful it was likely to be. I'll give it a little more light sanding to make sure I've got off what I can, and get on to varnishing!
I agree with not trying to achieve perfection. A lived in home (& its floor) needs to have character.
Some photos from the process - I haven't revarnished the floor yet, but everything else is done and it's looking pretty good!
Nice work @Chris_T! That looks very uniform after the stain. I'm waiting with anticipation to see the final results. You've certainly put in the effort.
Keep us updated.