For OP:
There is a very clear, but implied check on the 25th Amendment: if it's done against the President's will, and Congress doesn't agree with the VP and Cabinet, then after the 3 week period, the President is back in power, and while the President can't fire the VP, the President can fire any Cabinet member at will.
Step A. VP and half of Cabinet invoke 25th.
Step B. President disputes
Step C. House and Senate fail to get 2/3rds majority to remove President permanently.
Step D. President retakes power after the three week period.
Step E. President fires every Cabinet member who signed on to the 25th Amendment use.
This is basically inevitable and every Cabinet member knows this, which is the real check on inappropriate uses of the 25th Amendment.
While I agree with everything else, I believe Step D shouldn't be there; the relevant part of the 25th amendment reads as such:
If the Congress, within twenty-one days after receipt of the latter written declaration, or, if Congress is not in session, within twenty-one days after Congress is required to assemble, determines by two-thirds vote of both Houses that the President is unable to discharge the powers and duties of his office, the Vice President shall continue to discharge the same as Acting President; otherwise, the President shall resume the powers and duties of his office.
From my read of the amendment, if Congress voted and failed to support the Vice President's assertion of the President's inability the President would resume his powers immediately. The only mention of the three week period is in how long Congress has to vote on the inability, not in how long the Vice President would be Acting President. If the VP sent Congress the letter indicating presidential inability and Congress voted against the VP right then and there it makes no sense that he would get to continue as Acting President for three weeks.
