Is there any way to access the locked Feats in character creation if you already own the 2014 Player's Handbook? When I try to access "Mounted Combatant" it directs me to the marketplace and says I have to purchase the book to unlock. I don't need 2 copies - is there a way to register my copy and unlock this content on dndbeyond?

Nope.

Your options include:

1. Finding somebody who is willing to share content with you so you can add it to your character. This involves somebody with a Master tier subscription and enabling this on a campaign that you and whoever has the feat joins. The character you join with will be able to add it.
2. Recreating the feat for your own private use (i.e. do not share publicly) using the Homebrew tools.