"[ but does the foot have to
"[ but does the foot have to come off to only replace the exhaust bellow? ]
No, but you have to work very carefully to not damage the shift cable bellows, which is much more fragile. You could actually cut thru the exhaust bellows to remove easier. But replacing the bellows would be more difficult. To have working room you would have to have the unit somewhat tilted back, clamp one end, and try to get other end clamped while stretching the bellows to reach the second mount. Like, you would need three hands in there with hardly room for one.
But, you could use an "exhaust tube" which replaces the bellows (see links in right column to find bellows parts for mercruiser). The tube clamps only at the front and would be easy to put into position."