Well, look at the other people in A+. Green Lantern has a special that's a gigantic hand that connects standing or in the air and is safe at midrange. Nightwing has staff stance with big normals, like his D1 that outranges Harley's pokes (maybe B2?) and has 6 frames of start-up and is plus on block. How does Harley out-footsie that?I think she is higher then A tier, A+ from my perspective. Her footsie game isn't as bad as you put out, she has plenty of overhead/low stuff that can string right into TS. She even has a few starters from mid range into TS, or silly slide and more pressure.
I would agree with you about Aquaman, he's a turd. His trait is annoying for Harley's BNB., and his pokes are ridiculous.
As far as Harley's mid range game I think it can create a great pressure game with her air shot(hits overhead) and her cupcake bombs(hits overhead), or MB on either of those. Plus the threat of silly slide into TS, or into regular shot or up shot.
With regards to getting in on Deathstroke, he can push you back, but not for too long. Using Harley's air shot gives her the advantage to move in over his shots (if timed right). when you play enough Deathstrokes you find it's not that hard to time, and to get in. At mid range the air shot is a punish against DS shots(standing). Granted, I'm playing online and not against "pros".
Air shot is good but also unsafe if she's high enough to hit at midrange and doesn't combo into anything. Cupcake bombs are extremely easy to reaction block or stuff during start-up. Her overhead normals are flat out bad because of their start-up, they don't work against strong players and you have to start getting creative with her offense. Harley is all-around an honest character while characters in A+ tier start to get really gross. Harley doesn't make that cut IMO.