1. Would you consider this cheating?
Whether I consider it cheating is irrelevant. If I did consider it cheating, I wouldn't do it. If I didn't, it comes down to whether my partner considers it cheating, and I either abide by their wishes or end the relationship (and then go wild with the VR AI-waifu).
That said, I don't know that I'd enter a monogamous relationship to begin with. I just don't feel that I have the right to control who someone else does or doesn't have sex with, any more than someone else has the right to control my sex life. More on this in the next section.
2. How would you feel if your partner did this? What would you say to them?
I wouldn't consider it cheating for my own partner to do it, because my primary concerns with "cheating" are the health risks. If my partner is physically having sex with another human being, they could potentially contract an STI and pass it on to me. I don't want to have that risk put on me without knowing about it and agreeing to it ahead of time. No matter how realistic, this potential scenario is still happening in a virtual environment, so that's a non-issue.
I would ask my partner how they liked the experience. It would be a fairly normal conversation, akin to talking about a new video game or having a massage. Maybe I'd tease them a little.
3. Suppose that this "advanced" AI is created by cloning (taking a picture) of a real person's brain and uploading it onto a supercomputer. Consequently, the AI is very similar to a real person, aside from not having a physical body. Does this change your opinion?
Not really. Regardless of how realistic the AI is, it's still functionally no different from getting off to porn in terms of meatspace consequences. It's better for them than porn anyway, since the participant is actually interacting with a reactive human analogue. Maybe we're getting into camgirl/cybering territory, but it's not something that really bothers me.
4. Suppose that the AI actually has an operator employed by some mega-company who occasionally pushes some buttons dictating how the AI should behave from a remote location. Does this change your opinion?
This actually brings it even closer to the porn comparison, since professional porn is staged and planned out. The operator here would be like the porn director. I'd expect people to have less of a problem with this, but I'm a fucking weirdo.
5. Suppose that the AI can be downloaded onto a portable device (i.e. smartphone). Your partner now spends a lot of time chatting to the AI, even outside of virtual reality. How would you feel?
If my partner's screen time with this AI is substantial enough to conflict with my own needs for companionship with them, I would recognize that my partner is having a need met by this AI that I clearly am not meeting, then try to identify what that need of theirs is so that we can work together towards getting both our needs met.
If it's not interfering with my needs, and they're enjoying themselves, I don't see an issue.