1: The check valve is a small banjo union (miniature version of the ones on the ends of your brake lines at the calipers) at the very front, upper left side of the carby from the riders view. It is held in by a flanged 8 mm hexagonal bolt. It's so close to the radiator fins that it is advisable to slip a piece of plastic or cardboard sheet down between the carb and radiator fins to protect the latter if you are trying to undo it.
2: The tube from the vacuum bell housing goes to the higher of the two fittings, the tube from the "triangle" (this is a fuel enrichment valve) goes to the lower fitting.
3: Won't make much difference. It's just a dust shield to keep that area of the carby behind the cover clean (that "box" the choke lever is attached to).
In the second picture you can clearly see the hose and check valve/banjo fitting and its 8mm bolt to the left centre of the picture.
As an aside, note the differences in the fuel inlets for these two RE5 carbs. I think the first one is a later version (still appeared on the M series though) and the second is an earlier version.
The metering pump check valve at the carb. will allow the metering pump oil into the float bowl for mixing but should check or not allow say a flooding carburetor to back wash into the pump which could allow the pump to be damaged from lack of lubrication. To test, get a hand held vacuum pump and test it in both directions. I've been successful with cleaning a stuck valve with wd/40 or carburetor/choke cleaner. We always caution to add oil to the fuel until certain it's functioning. The new hose is still available from Suzuki as a GT750 item and watch that it doesn't get pinched in the metering pump cover!.