My understanding is that ZED fares are based on mileage from origin to destination and connections make no difference in the fare. The NWA rep I spoke to said she 'thought' that a separate ZED ticket was needed for each leg, but wasn't sure. Being that these tickets are for my parents, I really don't want them getting stuck somewhere because the agent won't let them on. So then, my question: Will a ZED ticket showing MSP-PHL be accepted for a re-route thru DTW??

Thanks!!

I think you do need ZEDs for the actual routing (leg-by-leg). Otherwise an extreme multi-leg, out-of-the-way routing could cost the airline more than the ZED fare covers. But I've only used them for one-leggers.

When you price the zed ticket over the phone, you can tell them if it's a non-stop, or via. I'd tell them the whole itinerary, i.e. SFO->MSP->DCA, and return DCA->DTW->SFO on NWA, for example.

When you pick up your zed ticket, that's the way it's printed. The price is basely on mileage as you mention, so I don't think a re-route would make any difference, if it makes you feel better

Most carriers would accept zed ticket that is in the same price region. DCA=IAD=BWI...

ZED tickets are valid on every airline that flies the route so you don't need a pocket full of them. Ex. If you are flying from LHR to BKK and your ticket is on BA, you can still fly on Thai, Qantas and EVA.
With regards to different cities: I had a ZED on BA from SIN to LHR but the flights were packed. The agent put me on Qantas to FRA no problem. From Frankfurt I had to get a new ZED on Lufthansa to ARN because the original ZED was from LHR to ARN and the mileage was higher from FRA.
The bottom line is that ZED's are mileage specific not airline or to a lesser extent city specific. I have used ZED's many times and found most agents very knowledgeable and accommodating. This was OUTSIDE the states though. I would think your average agent here would be clueless as usual.
Hope this helps.