“If the invocation of the 25th occurs, could the Republicans find a way to defeat it?”
Sure. If Biden contested it and it went to Congress, it would only take 144 congressmen or 34 senators to defeat it. It is actually MORE difficult to remove a president who doesn’t want to leave by using the 25th Amendment than by impeachment. You only need a majority of the House to impeach, though you need 2/3 of the senators to convict. To remove a president via the 25th Amendment, you need 2/3 of each House to agree.