There was plenty of confusion on names (IIRC in 1942 had to send a letter to the US clarifying them) but "Round-up" seems mostly to have been used for a general invasion of Northern France in 1943, not just raids. However this varied over time, especially after a full-scale invasion in 1943 was put on hold.
The old Sledgehammer plans were still on hand, but as I understand the attention was to the 'opportunity' plans like Rankin I, II, & III. Perhaps "raid" is the wrong word. The objective of the Rankin plans was to capture a under defended port. Not just raid it. A opportunity plan as it were. Hyperwar has the best description I've found of the Rankin plans & Roundup in general. If anyone has a detailed analysis aside from Michael Guimarras essay I'd like to know about it.
I've not seen it presented in this way before - sources? Fifteen divisions sounds a little low given that British/Canadian/Polish could have provided at least twelve.
My estimate drawn from assorted descriptions. Depends on the date, and the logistics support at hand, particularly supply transport. That includes how swiftly Cherbourg or some other port might be restored. There is also the question of when various formations were fit for combat. I may be a bit conservative here. The descriptions I have of the early COSSAC version of Op Overlord only ran out to 30 days, so I am unsure what Morgan & Co expected.