[2023] Can maxed player use Nastroth minor reset?

Short answer: No.

"Nastroth can reset XP in a skill of your choice to its minimum, provided the chosen skill doesn't exceed level 10 (or level 15 if Constitution)."

https://runescape.wiki/w/Nastroth#Minor

This feature is, essentially, intended for pures who have slightly overlevelled to correct that mistake, and consequently doesn't reset associated content (hence the level requirement).

Unfortunately, no, there isn't anything I can do to help you. Content developers are not permitted to modify individual player accounts, so that Player Support have exclusive control over account management.

You could try contacting Player Support https://support.runescape.com/hc/en-gb/p/contact to submit a lost item claim. I suspect trimming an undesired skill cape wouldn't qualify as a missing item, but eligibility is Player Support's decision (hence I'm unable to intervene).

No, unfortunately I am not able to intervene on an individual player's behalf.

I literally and deliberately do not have a direct line of contact with Player Support, so that all players are treated fairly.

Player Support require the account holder to contact them for any discussions about your player data.

I can only point you toward their contact page, and wish you the best of luck.