what makes you suspect the Sinemas and Machins of the world are being “fall guys” instead of just being genuine blockers

They're granted committee seats by the party leadership in places where they can do the most damage.

Sinema's a freshman senator with a seat on the appropriations committee. That's unheard of for Junior senators.

Manchin's seat on the Energy and Martial Resources committee has given him a voice in pro-coal policy making for decades.

How did they get these positions? Schumer assigned them. He's endorsing their policy as a result.