@All: The last 4 weeks I took a break from forum work. I thought a lot about the C4-SFX and already startet to make changes on the drawing to support 280 AIO in sandwich and in classic layout 
I think the new revision will peak out at 11.2L - 11.5L. I will give you more details as soon as possible.
Funny to see: If you read the last pages and saw Optimums Twitter poll you see it is the right decision to made a case that does support sandwich or classic layout in one product. There are so many supporters on both sides.
Here are some answers to all the questions in the last weeks:
Yes sorry this decision is made.
Using very thick panels would require to change the manufacturer that does support extrusion and CNC milling. I am very happy with the quality of the pop-in panels.
I found a solution to support 280mm with clips in your prefered configuration.
Thank you for your feedback. I also saw your picture of the modded M1. I think you would aggree that a case like the modded M1 will not work because it requires watercooling the GPU to be 1 Slot. A case with more space for the GPU will have a height of ~260mm for 2-Slot GPUs and ~280mm for 3-Slot. This will add too much volume. But I have a solution for you. The new default position of the C4 will be GPU on top in classic or radiator on top in sandwich. If you now assemble in classic layout you can install a 280 radiator with slim fan above your single slot water cooled GPU. I know this is not exactly what you mean but it comes very close.
It will support 280 rad in sandwich mode.
I think your sweet spot discripes the new Cooler Master NR200.
Thank you for your feedback I also though about this. I think of adding 3mm to the width (from 148mm to 151mm) than the strix cards will fit and also these traditional water block:
Phanteks Glacier (120mm)
Corsair Hydro X (121mm)
Watercool Heatkiller IV (118mm)
Alphacool Eisblock (120mm)
I am thinking of changing the outer design to that one of the old C4-SFX with these cutouts next to the front on each side. This would give room for I/O ports on the sides. Maybe I will not consider 2x Type-A but maybe 1x Type-A and 1x Type-C. Keep in mind only loud thoughts.
And you will get 3 Slot support in sandwich and classic layout. I think of adding 3mm to the width (from 148mm to 151mm) than the strix cards will fit.
First question: From the sides and top
Second question: If you mean with flipped the sandwich version. Yes you will be able to mount a 280 or 240 rad here.
It will only fit in sandwich layout. And it does also fit in the A4. Check out this build https://pcpartpicker.com/b/rZRJ7P
If I missed a question please let me know!
