Does MMO (limiting Mach number) assume any maneuvering limits?
The problem is that in a deliberately entered spiral dive it is very difficult to keep things in check.  In an idle descent, pitch is your only speed control (well... really, in the end, pitch is your speed control all the time; any glider pilot understands that), and in a spiral dive trying to slow down by increasing pitch is trickier because a lot of the pitch input just steepens the spiral and you just pull more G instead of slowing down.  Which is the whole reason spiral dives are to be avoided in the first place, especially in the IFR world.

In a jet in an emergency descent you will generally dump flight spoilers with the engines at idle and push over to accelerate to MMO/VMO.  An attempt to maintain MMO/VMO in a spiral dive turn would probably end poorly.  If you are diving straight at VMO, spoilers out, and want to get down faster, depending on the airplane you might get a faster descent by slowing to gear extension speed and extending the gear, but I don't think most manufacturers recommend that and it's not something I was trained to do (on RJs).

Sideslip doesn't come into it as sidesliping swept wing aircraft deliberately is very dangerous (it doesn't take much skid/slip to make a swept wing airplane want to roll on its back), and nasty dutch roll situations can result.  In any jet, once you are off the ground, the yaw damper system takes care of adverse yaw and any random disturbances, and you never touch the rudder pedals unless an engine quits.  It's generally feet on the floor after the initial departure profile until final approach.

In any case, in an emergency descent you are way above maneuvering speed so pulling too hard will fold the wings before stalling AOA is reached and trying to get more descent rate by spiraling would be suicidal.