I'll add that this has happened several other times in our history (where a president's health has declined to where it isn't obvious who's in charge). We really should have an amendment to avoid it from happening and prescribe a process to remedy. Perhaps congress could have heightened impeachment power.
Discussion
We do have the 25th amendment though...
The 25th is more for if the president is fully unconscious. It doesn't work if there is a credible resistance by the president as has been shown for the last few weeks. From the people I listen to the 25th amendment talk was a red herring.
I agree with everything you're saying. We also can only guess at this point. Will we ever know what's really happening?