She just responds very slowly or partially follow commands like on "sit" she will start to sit then stand again.
It sounds to me like she is not entirely certain what it is you want of her. Bear in mind that she's gotten the same commands in a specific tone of voice and/or with accompanying specific hand signals for a pretty long time, so that is what she's used to. If you use a different tone of voice, different body posture, or different hand signals, you may be sending her "mixed messages", which is why you're not getting the behavior. (Kind of like when she would be slow or not listen to Brian...)
I don't think it's "resisting" - it's being unsure of what you want. A class would definitely help you communicate better with her. But I think a lot of it is just taking the time to work with her (in a class or outside of it) so that she learns your handling and your commands and gets used to them.
What I would do on the sit, for example - if she does start to sit but then gets back up, I would not reward/praise her until she sits. Either wait a little for her to go back to a sit (if she's trying to figure out what it is you want, she should offer behaviors). If she does not offer the behavior because she's confused, make it clear what you want. You can either step toward her so she has to plunk her behind down, or use your hand and just push down on her behind just a little when you tell her to "sit", then praise/reward her for sitting.
maybe even agility once you understand how to communicate with Abby
If there is a "ground work for agility" type of class where they work on communicating, that would be a great choice. An actual agility class probably not so much, considering her arthritis.