Guys:
I think going back to 5-8 June was the right idea & I have some thoughts.
Dates: 5 June was the original date, and even with the delay for wx it became the load & move day. 6 June is obviously THE day & no it is not just a few things, we had 4 divisions (IIRC) and a bunch of support stuff on the beach. Omaha was still being opened on 7-8 while the other beaches had made more progress.
Terrain: There are several good maps/diagrams that mark the boundaries of the fleet movements from ports to the Normandy shore, and which mark the limits of advance within the date ranges given.
So:
-- For Military(armor, figures) & Naval, you had to be within those boundaries & dates.
-- For Air, it is a bit more difficult, but I would say that it is pretty easy to determine those units who were in direct support of the invasion, to include interdiction outside the boundaries above.
IMHO the net from a group build is for people to learn something. One of the ways to learn something is for discussions of what is/isn't allowed, with references not just "I thinks". That way everyone learns something about the event.
Having said that, lets look at an idea that came up -- Deception. If the deception plan was actively running on 5-8 June (don't know) and was in the terrain boundaries suggested (probably was), then it is legitimate "D-Day" topic.
In a similar manner, a stretch could be made for the Tigers if one considers that the French Resistance & Air Interdiction slowed the arrival of the German armor once released (when was that?) and this was important to the success of the landings/operation, so a diorama focused on stalled/delayed/harrassed German units might be appropriate.
In these two cases & possibly others, it is up to the proponent (guy who wants to build) to lay his *FACTS* out & then it falls on the GB "owner" to earn his money by making a decision based on those facts and any input from others.
The above is mostly written from an Allied perspective, but the German builders can do same thing -- were they in the area on the dates, etc ...