Un-matching Bridesmaids

I would just stick her at the end and call it a day. With her being farthest away, she'll likely be framed out of many of your ceremony photos.

A shall in the same color of the dress would help make it "appear" more similar to the others. (how rude of her to take it upon herself to overrule you!)

If you want to let everyone stick with their existing dress, you could give all your maids coordinating necklaces, and give the MOHs the fanciest, most bejeweled necklaces. You can also have your florist make the bridesmaid bouquets different sizes.

I know you said you didn't want to be confrontational, but I would absolutely your ground and let her know the dress she purchases was not one of the options provided. You invited a BM into your wedding party as a favor to someone else and she's being the most entitled stick in the mud. There is no reason to be more lenient with her than your other bridesmaids. If anything, she should be doing MORE to try to appease you.

The dress you picked out is probably one of the most universally flattering styles that exist, and the one she picked is more revealing. If she went for a more conservative style (long sleeves or no slit) I might think she was trying to hide an insecurity but I think she may actually be trying to make herself the focus on your day by standing out in a more attention grabbing way.

Alternatively, you could consider expanding dress options so that all of your maids have slightly different styles, that way no one dress stands out much more than the others. I personally love mismatched bridesmaids dresses, but you'd probably need all three adults in three distinct styles to make it work.

I would probably tell her to get the dress I requested or step out of the party, but I'm probably more confrontational than you. I absolutely would not stand to be bullied by my brother's sister, or any sort of BS from someone I didn't really want in my wedding party anyway. And I wouldn't feel any sympathy for her buying a dress without checking with you first, or let that guilt me into letting her wear it if it wasn't what I wanted and didn't go with the overall aesthetic.