I will do my best here.
I take it the reason you are asking about using NM wire is to save money?
Purchasing NM is much cheaper. You can still run NM inside the conduct if there is
room left. So if your conduit is 3/4 inch. you maybe able to pull 14/2 wire through with Lube.
Your subpanel would need a separate ground bus since the conduct is considered the (Ground)
By installing a Ground bus and using the NM 14/2 or 12/2 you must connect the Neutral and Ground separate as for the panel is not bonded. I believe the term is floating ground and neutral. Your Subpanel is connected to your Building since it is not Bonded. The question i have is why is your panel has only 1 bus bar? There should be 2 one for the Neutral and the other for the Ground. Your subpanel should have 1 Aluminum service line that contains 2 hots leads and 1 neutral and 1 ground. The other question i have is the service line coming from the Main Panel inside metal conduct as well?
The gang boxes are grounded by the Conduct, but I would recommend you put a Pig Tail in the box.
Since there is still some uncertainty here, I took a minute to create an image that might fix some of this.
Lightning is a DC shot of electricity going to the ground, it's source. So it's easiest to consider it that way. Next, lightning is so powerful that it creates a surrounding zone of energy on any conductive material that is able, the yellow circles. Everything that is within a distance becomes energized to a point, but is again still from the lightning and is trying to go one way - to the ground. This is why everything metal is bonded or grounded: so that nothing needs to arc to find a path, since it has it's own. Regardless of how it gets to ground, it's going there.
Back to the electrodes though. Consider my image above as being of a main structure and a garage. For all purposes necessary with lightning, there is no need to tie them together. As mentioned above about 100,000A of power coming off of a lightning strike, more rods and such can help dissipate the power faster and catch a little more of it than is going through metal in your home, but regardless it's still 100,000A of power ... what happens, happens.
Whether it's your garage or your neighbors house that is splitting the distance of the strike zone, tying them together won't fully relieve the problem of getting struck by lightning.
For purposes of the code, which you fully understand, the connection that is made between the two of them is your EGC that is only for ground faults. The reason this is in the code is because some people think that running a ground rod will be the miracle solution to ground faults and that the wire isn't necessary. This isn't the case though.
For ground faults, the EGC is the answer because it sends the fault back to it's own source - the utility. For small voltage differences throughout a system, a ground rod (or multiple for higher systems) is driven. However, for lightning, you are again giving the ground rod as a path for the voltage difference caused by it, but tying the full system together doesn't help all that much more than one rod.
Hopefully this answer helps a little better.
Best Answer
If an outlet has a ground terminal, it should be grounded. No ifs or buts. (And IMO, giving the illusion of ground protection is even worse than not having any protection.). I don't think you can even get two-prong outlets any more, so I think you'll have to fix the wiring. It sounds like you're still negotiating, so this is a good bargaining point. And if you don't get it fixed now, when it comes time to sell, any potential buyers would be within their rights to make you fix it then.
An easy way to check that your wiring has a ground conductor is to open up the service panel, look for the breaker that controls those outlets, then trace the black-shielded wire to where the cable feeds into the panel. (If it's present, ground is unshielded.)
If you have grounded wiring at the service panel, the next thing to check would be to look inside the outlets in question to see if the wiring there is grounded (it may just be a loose connection, and since it's common to daisy-chain outlets in a room, one faulty wire could affect several outlets).
If you have don't have ground at the outlets, but did at the service panel, you probably have a junction box joining the older ungrounded wiring and newer grounded wiring. Time to go exploring in the attic or crawl-space / basement.
GFCIs detect a difference between current going out on the live terminal and current coming in on the neutral terminal. If you're the easiest path to ground in an electrical fault, a GFCI will detect that and break the circuit; without a GFCI, you'd have to hope that you draw enough current -- and for long enough -- to trip a regular circuit breaker. On the whole, though, I'd rather that current was running to ground through some copper that's intended for just that purpose.