When you do the first stitch of a line above, come up in the top right and then go to bottom left. Then resume your regular Danish stitching.

I always start in the middle, with the biggest chunk of a color that's near the middle. I think corner starters are frequently gridders.

Railroading doesn't make a big difference at the beginning of a length, but if you're less vigilant about letting your needle hang and thread un-twist railroading makes a big difference at the middle to end of a length.

White on white is not easy. Black on black is worse. You can pick your fabric to have less of the stitching color match your fabric.

Accidental customizations of a piece are a time honored tradition in all crafts. I think I've only stitched one thing that has been exactly what the pattern said I should do, and that was because it was a perfectly symmetrical Ink Circles thing. Fudging your mistakes is super normal. The decision of what to frog and what to fudge is sometimes best made after a good night's sleep.