[Bubba_Leroy]

A casual reading of the 25th Amendment seems to indicate that any action to remove a President for inability has to originate in the Executive branch, with either the President or the Vice President.

That is correct.

The only Constitutional method for the Legislative branch to remove a President is impeachment by the House and conviction by the Senate.

[Vigilanteman]

The only Constitutional method for the Legislative branch to remove a President is impeachment by the House and conviction by the Senate.

Not only. See my link at Post #42.

This method is actually more difficult, however, since impeachment only requires a majority vote of the house and conviction a two-thirds vote by the senate.

The alternative method requires a two-thirds vote by both houses.