Is shoreline a must when searching for spawning flats? Or is it possible to have flats away form shoreline if cover is present on a large flat off the bank?

No, grass beds located away from the shoreline

Also in coves (depending on depths) bass can spawn in the middle of it.

Do not know if that answers your question or not, but hope it helps.

Shallow humps (6-10') that have some protection from northern winds are a good bed for bigger fish as well. And since these fish are off the beaten path, not many anglers will find them and they will be easier to catch.

Most bass spawn in fairly predictable places given the following criteria:

Solid bottom: Bass avoid soft, muck bottoms; instead they prefer to spawn on hard sandy bottoms, gravel or rocky banks, large boulders or even a fallen log or lily pad root.

Shallow water: The fertilization and hatching process requires the warmth provided by sunlight penetration; which is why the fish migrate into shallow water (with the exception of deep, clear western reservoirs). Bass will bed generally in a foot to 6 feet of water

Protected areas: When possible, bass will bed in water that is sheltered somewhat from the elements like in a cove or pockets.

However bass in some lakes are known to spawn in open water at depths down to 10'+ on tops of stumps.

Perhaps of interest:

http://www.bassresource.com/fishing/springflats1.html