My LO was put in hospital because of poor oxygen level from pp MC facility. What rights as his guardian do I have in this case?

I too am confused why you are asking YOUR PCP and why you think your PCP has any say in this? Your dr has no businesses deciding what is right for your husband if your husband isn’t his patient. It is ultimately up to the memory care not your dr or any other dr and if he now requires oxygen support, the MC may rightfully feel his needs are too great for them. Are they willing to take him back after he goes to rehab? Because that’s what it sounds like you are saying. Also sounds like he wouldn’t benefit from rehab and I agree there so you will probably have to speak to the director at the MC and try to work something out.

It's not clear to me whether or not he went from memory care to rehab b/c of a pulmonary and oxygen level issue, or b/c the MC staff initiated the transfer for other reasons.

And has he gone, or is this still pending?

To me, the issue is his oxygen level and pulmonary conditions as a first consideration, above what the staff want b/c he may be a challenging patient in their minds.   

As to your rights, I think a discussion with the MC DON or other high level nurse would be appropriate to clarify the issues, then you can make a decision.   Has YOUR (??) personal doctor examined him?    If he or she has and feels that memory care is the appropriate setting, go with that.  But be sure the issue of adequate oxygen levels is addressed.

And as his guardian, I assume that his care and needs would pre-empt the desires of an MC staff.   What does the guardianship order provide?

He entered for poor Oxygen level. Unless he is DNR/No Code, it is unlikely that the MC will accept him back. Even then he would need hospice for extra care. They will not be able to care for him adequately for this condition unless it is expected/accepted that he may be dying. I think that rehab also may refuse him. He does not sound as though he can participate in rehab, PT and OT. Sounds more as though he will need SNF (skilled nursing facility).
As a wife you do not have the ability o act as a physician. You say YOUR personal doctor has said....................but your own doctor has nothing to say in this case. Likely his own doesn't either. It will be the hospitalists at the hospital who will decide what level of care is medically appropriate now. You can decide only if you are his POA and he is dying and a DNR. In fact, even then the MC can say that he requires more care than they can provide and insist on SNF placement.
I am so sorry you are both going through all this; it must be unbelievably difficult.