Neutral wire is tied to my electric metal box, why is it so?
I would use the white as a neutral

Based on what we can tell so far, the white is indeed a neutral wire, so that should be wired up to the silver screw on the outlet, with the brass screw on the outlet connected to a pigtail to the existing black hot, which is also pigtailed to the switches.  It definitely should not be landed on the box, though!

And run a separate ground wire

From there, you can then run a separate ground wire (green #12 THHN or bare #12 copper works, provided it's not subject to physical damage) back to a suitable grounding point (i.e. another suitably sized equipment grounding wire, the wire that connects the panel to the grounding electrode system, or back to the panel, but not to a water pipe) to ground the box and receptacle.

As to the rest of the improper grounds...

To clean up the rest of the improper grounds, what I would do is take some 6AWG bare copper and run it alongside your cold water pipe from the point where the water pipe is connected to the panel to the end of the water pipe run.  This will serve as an equipment grounding "busbar" or "trunk" of sorts.  Then, I would use an Ilsco GTT-2-2 tap connector to connect the grounding electrode/bonding conductor (the wire from your water pipe to the panel) to your grounding bus wire -- the part that comes off and goes back on lets you slip the existing wire in there without having to cut it and splice it back together, while your new wire can go in the other hole, and both screws can get tightened down then.  From there, you can use split bolts, or a similar sort of tap connector, to connect your existing grounding bus wire to the improper ground wires after disconnecting them from however they were attached to the water pipe.