Freddie de Guingand?
He was a staff officer with zero command experience at division level or higher. He would never have been appointed army group commander. Think Paulus in khaki....
Wavell would have been a non-starter. He was too deeply embedded in India and wasn't fashionable and considered too old-fashioned. Its rather doubtful he could have coordinated a modern mechanized army and fought succesful against the Wehrmacht. Wavell was more at home against unarmed tribesmen and the Italians.
Auchinleck was another "has been" by that point. Unfairly perhaps but he didn't have the credentials any more to be allowed to face the Germans in Europe.
That leaves Brooke and Alexander. Brooke might have liked the job and he probably would have done well. Although he certainly lacked experience as a field commander, having only briefly commanded a corps in the field. However, it is moot because he would never leave London. He was the only man able to deal with Churchill and with him gone, there would have been numerous bloody brain farts like Gallipoli and Norway.
Which leaves Alexander. A competent man perhaps best suited to the role. He was able to work with the Americans and he allowed his army commanders a lot of leeway.
The main reason Freddie was a candidate was not because he could do Montgomery's job efficiently or whatever but because he was a very diplomatic man who the Americans liked. It was the height of Anti-Monty feeling amungst the Americans and in SHEAF that led to de Guingand being considered a possible replacement. It was possible but highly unlikely because of de Guingand's lack of command experiance and he was very unlikely to accept it anyway.
As to the others, I agree with you on Wavell, Auchinleck and Alanbrooke but not on Alexander.
Alex was the British Eisenhower. Great diplomatically and politically but not all that good at the military side of his job and relied heavilly on Montgomery, Leese and McCreery to do most of the work for him. Alanbrooke and Monty agreed that Alex, though personally brave and very nice man and very easy to get on with, had no aptitude for strategical or tactical thinking and was always influenced by whoever talked to him last.
To put it bluntly, Alex was a pushover.
The American's liked him because he was a pushover who let them do whatever they wanted and never did anything to get their bad sides. Had he been in given the Ground Forces Commander role for Normandy he would have let Freddie Morgan walk all over him, the OVERLORD plan would never have been created, he would have bowed to any of Churchill or Eisenhower or Tedder fears and changed the plan repeatedly and increase the chance of failure.
Alexander would have been a complete failure as 21st Army Group Commander in everything but relations with the Americans.