At the beginning of September 1944 the British 11th Armoured Division captured the port of Antwerp with its dock facilities intact. Its use however was delayed by the Germans retaining control of the approaches to it.
If the Allies had been able to take control of the Scheldt Estuary as well as the port, and had been able to immediately start using it, what effect would this have had on the supply situation and what would this do for the Western Allied advance?
In OTL after the failure of Market-Garden the allies returned to clearing Antwerp approaches in early Oct 44 and it continued until 8 Nov. The mine cleanup took three weeks and the first ship arrived on 28 Nov. I also remember reading that the first four weeks Antwerp only manage to offload some 10.000 tons a day (roughly as much as coming by train from southern France). Only in the end of December did Antwerp open fully to provide 40.000 tons / day. However by that time the Arden counteroffensive stalled the allied progress which was renewed only in January 45.
In the ATL it would probably took just a few days to secure accesses to Antwerps as they were by then empty and German forces returned only with withdrawal from the Pas de Calais area. Therefore it is possible to assume that by, say, 10 Sep the approaches could be clear for demining. That would open Antwerps for limited operation by the end of September and since the end of September / early October the Antwerp could be running at full speed, capable of supplying all attacking forces in the west.
Unlike in OTL, when untill the end of 1944 the allies only managed to supply a limited number of attacking forces, in ATL there could be a broad offensive by mid - end of October. By then allies would have in France some 60+ divisions of which 30 could be used for attack. That was roughly twice as much as in OTL, when allies attacked mostly only in the Hurtgen Forrest.
With 30 instead of 15 divisions attacking on a broad front by the end of October 44 it is likely that Germans would have to use the forces reserved for the Ardennes for stopping the allied onslaught, so probably no surprising counteroffensive slows the allies down.
How long it would take to reach Rhine I can't say as the allies in October 44 would be some 25-30 divisions weaker than the allies of March 45.