[rolnor]

How does one determine pKa of a amineoxide under anhydrous conditions? I really dont have any clue about this, I have searched in the literature but cant find any relevant answer.
I did a new search, you can do it spectrophotometric in DMSO;
https://pubs.acs.org/doi/pdf/10.1021/ja00857a010

I am not sure that you can do it with such a strong acid as a protonated amineoxide though?

[phth]

Should be fine. I would read about metal hydricities because I think you can find the answer to stability concerns in the literature. A review article that I quickly googled: Thermodynamic Hydricity of Transition Metal Hydrides https://doi.org/10.1021/acs.chemrev.6b00168 

[rolnor]

I dont understand your answer at all, I am talking about amineoxides?