To try and answer your questions

The bimini should be strong enough to use when sailing, although there are obviously limits

It should cover the helm and most of the cockpit, particularly if you have a table and want to eat outside - although recognise there will be times, for example when moored EW when the sun will come in either morning or evening.

It should be usable with the sprayhood up - they perform different functions.

Constraints in design are

Head clearance at the forward end. To fit under the boom, may be too low when coming up the companionway. This also introduces compromises when trying to link up to spray hood to make enclosed cockpit.

Clearance aft, particularly around backstays, and having enough headroom to get through transom gate.

Folding in such a way that there is still headroom for helm.

Using a radar arch for the aft mounting is a good thing in general as it results in a more secure mounting. However introduces new problems such as does the arch sweep back or forward and what else you are mounting on it.

Refinements include drop down sides and aft to provide additional shade or enclose cockpit.

If I were starting from scratch, I would try to design it using a radar arch, but the other details would depend on the specifics of the boat. I would avoid if possible having the backstays go through the cover, but this requirement often conflicts with giving cover to the helm.

All you can do really is have a look at as many designs as possible, particularly for your boat and find someone who understands what you want. Also recognise it will never be perfect.

Hope this helps