Replacement will be either Brooke or Marshall. Neither of them had much use for Monty. Marshall is more likely, especially if it is after Paris falls.
I'll contend the point that Alanbrooke didn't have much use for Monty. Alanbrooke was Monty's biggest supporter and certainly held him in high regards as far as his abilities in the field of battle went - if not so much for allied relations and keeping his opinions to himself. Monty will not face the prospect of a diminished role if Alanbrooke replaces a deceased Eisenhower as he will still be retained as the commander of 21st Army Group under Alanbrooke who would be SHEAF.
If Marshall gets the job then Monty's role will be significantly diminished and Marshall will favor the American Generals. Marshall had no real respect for Montgomery as a general at all and reports from the American's in the field certainly coloured his views towards the Field Marshall (started when Eisenhower played down Monty's achievements in North Africa and continued since).
In an Alanbrooke getting SHEAF command scenario I would imagine that he would halt the broad front and form a strong force with which to launch a more concentrated thrust against the Germans. He was, after all, one of the biggest critics of the Broad Front strategy and believed it had significantly lengthened the war.
In a Marshall getting SHEAF command scenario then I imagine the Broad Front Strategy is continued as Marshall supported Eisenhower's strategy wholeheartedly and the fact that Eisenhower did tell him that it was going so well means that I dont see him changing the strategy at all.
In a Bradley getting SHEAF command scenario then the strategy is changed to support a concentrated thrust in the Saar region with 21st Army group being used as flank guard for 12th Army Group. This is difinitive as that was what Bradley wanted to do in OTL.
In a Monty getting SHEAF command scenario, unlikely though that may be, the single concentrated thrust is resurrected and the North is given priority over the rest of the front with the Rhur industiral valley as its first aim and eventually Berlin.
Other potential SHEAF officer are Alexander or Clark but neither would be in the front running positions for that job.