If my information on the condition & deployment (or chaos) of the 15th Army 4 - 11 September is correct a set piece amphib op with specialist units may not be necessary. If the airborne operation captures the 2 or three primary ferry sites the 15th Armies crossing plan is trashed & they will have to improvise, while being overrun using whatever other boats they can bring up the coast and canals. The only actual garrison I can identify on Walchern Island on the 4th are some artillery men at a 'fortress' position on the west end of the island & some anti aircraft units.
Defences of Walcheren (which was designated an Atlantic Wall Fortress) include:
6 batteries of naval guns, each at least 4 guns in 94 - 202 mm range
17 batteries of smaller close defence guns along the coast
7 heavy anti-aircraft batteries in the interior
89th Fortress Regiment
1018th and 1019th Regiments of 70th Division each with 3 battalions.
Two of these regiments were sent south of Ghent "in the first days of September" to reinforce 15th Army, but I assume they would still be in Walcheren or Breskens on the 4th.
All from Moulton's
Battle for Antwerp.
FAAA refused an airborne landing on Walcheren, the specialist naval assault and close support vessels will still off Le Havre until it fell on 12th September; the landward approach was along a narrow km+ embankment - it is very difficult to construct a scenario where Walcheren falls quickly or cheaply.
However taking Breskens and the ferry sites stops 15th Army crossing the Scheldt and interfering in Market Garden.
I'd expect Eisenhower would endorse this operation readily as it fit his messages to 21 AG to give opening the port of Antwerp priority.
Eisenhower didn't give Antwerp
priority at the time
, it was just in his list of targets which IIRC also included the Saar, Ruhr and Rotterdam.