Going back to the OP, how come no one has mentioned Monty tripping and breaking his neck going down a flight of stairs around February 1944? That alone should put the Allies well ahead of schedule...
Before Anglophiles jump in, note who failed to take Caen on time, who interfered in fully closing the Falaise pocket, who decided Antwerp wasn't important, and who masterminded Market-Garden.
Caen wasn't taken on time, sure, but were the other objectives Montgomery made for that portion of the campaign met? Were the majority of German troops drawn to, held down and written off in that area? Were the Allies across the seine by Day 90? The answer to those two questions is yes. I have never understood why one tactical failure is so repeated held against Montgomery when his strategical objectives were all met.
Bradley was the man who prevented the closing of the Falaise Gap. It was Bradley who, with any consultation with Montgomery, explicitly ordered Patton to go no further north than Argentan. Patton wanted to further north and Montgomery had set Chambois as the meeting place of the Allied pincers. But Bradley was afraid that there would be friendly fire if he allowed Patton to go north and also thought that Patton wasn't cut out to face a proper German army. Bradley said he prefered a strong shoulder at Argentan to a broken neck at Chambois.
In addition, once Patton recieved his orders from Bradley not to go any furthern North than Argentan he suddenly decided that finishing off the Germans wasn't worth his time and he was going to go for Paris.
Two weeks before submitting the MARKET GARDEN plan Montgomery submitted a plan to SHEAF which would see First Allied Airbourn Army used in the clearing of the Scheldt. This was vetoed by Browning and Brereton who saw their Army as an advanced tactical strike force, not something to be used in work to the rear. In addition Eisenhower never gave the clearing of the Scheldt any priority whatsoever, so it was hardly just Monty's mistake.
The planning of MARKET GARDEN was primarilly done by Browning and Brereton. Montgomery supported it and came up with the initial concept but didn't have a massive part in the actual planning of the operation. That was one of the reasons for its failure.
Considering that Eisenhower said something along the lines of "no matter what they say against him, Monty got us there" in regards to Operation OVERLORD, and Walter Bedel Smtih echoes similar sentiments and Bradley - a great Monty-Basher in his own right - also praised Montgomery as being essential to the victory in Normandy, I dont think his death would make the operation go any smoother.