The most likely option appears to me at the moment to be sticking with Biden, running a monster "he's OK" gaslighting campaign in the media, and cheating like hell where they can get away with it. The real problem is not the Democrat candidate, it's one Donald Trump, and I hope his security is up to the job.
A 25th Amendment action would result in a President Johnson, once evidence of Kamala’s ineligibility for her office is verified by Congress.
That’s the way I read sec. 4 also... only need Congress to act if President disputes his incapacity.