Fluid Steel Krupp Essen identifies the steel used in the barrels and the steel company that provided the barrel blanks. Krupp itself did not make small arms or sporting guns.
If there is no maker's name, it is likely the product of the so-called "guild system" of small shops, where one shop made barrels, another actions, and so on, while yet another shop fitted and assembled the parts into a functioning gun.
On the various proof marks you are essentially correct except that the crown/G is the proof for rifled barrels, not specifically for a solid projectile. The shotgun barrels are 12 gauge (a bit unusual if the gun were made for sale in Germany); the 7,8x57 would indicate a rifle barrel of 8x57JR or 8x5JRS, probably the latter at that date. That would be the rimmed version of the German military cartridge.
There are two problems in trying to evaluate the gun. One is its "no name" source, but worse is its condition. It appears to be fairly heavily rusted and pitted, to the extent that I think any attempt to remove the rust would ruin the appearance of the gun. Ordinarily a gun like that could bring $5k or more; in its current condition, I would think that gun might bring $1200.