Yes... those books. Griggs book is sadly misstitled. It is a nice readable primer on the Symbol conference @ Casablanca in January 1940. In only one chapter does Grigg directly address the subject of a 1943 invasion. The other 90% concerns the debate over Allied strategy during 1942 & the final decision not to invade NW Europe in 1943.
Dunns book has been criticized for playing fast and loose with the numbers. ie: he is accused of using the total number globally for LST at the end of 1943 as the number available for use from the UK in mid 1943. Since I have only skimmed his book and dont have a copy I'll leave this one for others to judge.
Most other analysis I've seen of the question have been generally weak in facts. Or lacking in depth. ie: Post war Montgomery is to have said it was a mistake for the Allies not to have invaded NW Europe in 1943. Since he was involved in such planning in 1942, before sent to command 8th Army, and was involved in the same planning 18 months later as 21st Army Group commander this statement carries some weight, even tho I've not seen supporting argument from Monty.
There are a few analysis of the question that support the idea for a 1942 attack. Michael Guimarra provided a long essay on the subject. Atkinson is a little critical of Brookes arguments against a 1943 attack.
A decade ago I set out to study the difficulty or impossibility of a 1943 attack into NW Europe. That resulted in a sort of 'Oxford Movement' moment for me & in the end I found the difficulties most other folks raised were not in fact supportable, and the real problems were not what most folks considered. Even in the cases where I had some questions it was more of a problem that the evidence either way could not be found. I am still hoping it can be found, but am not optimistic.
So, looking at the OP concerning a 1942 attack.
Pretty straightforward. If the Eastern Front falters and the WAllies launch Roundup on the French Coast, what would happen?
A lot depends on exactly when this occurs. In March, in June, in October? Allied strength and ability varies, & not always in ways folks assume.
What is the chance for sucess/failure? What might the casualties look like? If it fails, how long before the allies launch a second attempt, and would they change the location from the English Channel?
At this point I'd refer to Guimarras essay
'D-Day 1942, D-Day 1944:A Comparative Analysis of Operations SLEDGEHAMMER and OVERLORD' Trying to cite from the many pages would be a exercise in futility. Better the interested parties read the entire thing themselves.