Sanding sealer is fine for wood with smaller grain holes to fill, but for the larger grain patterns in ash, then a proper grain filler is easiest. Otherwise you'd need to apply many coats of sanding sealer to do the same job.

It's never easy knowing what size holes to drill for posts. You don't want to make them too large so the posts re loose, yet too small and you run into the problems you've encountered. A hole about 0.5mm smaller than the splines seem to work for me, at least with 12mm diameter posts. If there's a section of the post at the bottom with no splines on it that's a smaller diameter than the splines, then that should be the ideal hole size.

But good news that you've managed to fix the splits and other small accidents. I'm hoping that one day I'll have learnt enough from my mistakes to build at least one guitar without any mishaps and with everything going as it should. I'm not holding my breath!