Is there any trick to sanding the fretboard between the frets for maple? Or just patience and a small thing to use as a sanding block? It's quite hard to sand with the grain on the narrower frets at the dusty end.
I typically don't sand a (fretted) maple fretboard unless there is a problem (obvious rough spot, fret marker not seated properly, etc). If you feel you do need to, try hitting it with either a fine synthetic sanding pad or steel wool. (FTR, I never recommend or use steel wool, but that's just me and plenty of people do).
With s/w or sanding pad (extra fine-fine) you can go across the grain without leaving obvious whirls, and it won't gouge the frets like sandpaper will.


It looks to me like the maker nibbled off the tang at the ends of the frets, as if they were going in to a bound fretboard, and then put filler in to the resulting gaps.
That's how it done. The tang is cut back even on non-bound fretboards. Usually only about 1 mm though. Just enough to securely hold some filler in there.

Especially for the fourth fret, which is at least a mil short on the treble side:
That definitely is not the way it should be, but it may be ok. You'll have to see where the E6 string falls when you do you're alignment. Then decide if it's going to be a problem when playing. Worst case is replacing the one fret, but I wouldn't say you have to go that far yet.