Congrats! To answer your questions:
It depends how close it is to your erotica niche. I would say you might as well use your existing pen for the transition into romance, since you can always make a new pen later if you want to. Lots of people use their existing pen, learn how to consistently rank well in romance, and then start a new pen down the road. But if your erotica is really niche or has a chance of turning off some of your readers due to being too different from your romance then maybe consider starting a new pen. Ultimately it's a personal choice each author needs to make.
I never promoted mine so I can't really say. If I were you I wouldn't worry about promoing shorts, it's never seemed worth it to me. For romance Facebook can be good if you join groups relevant to your niche. A mailing list and consistent publishing will always be the best way to get sales, especially with shorts. If you're still curious about paid promos I would suggest signing up to their newsletters and seeing which books they promote, then checking the ranks of those books and seeing if the promo did much for them. That way you could find works similar to yours and see how they perform and if the promo is worth paying for.
Personally I don't think it is, unless you're really bad with photoshop/GIMP/canva. I have a feeling your covers aren't as bad as you think if you're making this much off 13 titles. However it might be worth it for a romance novel just so you get the strongest launch possible.