You know, I have always liked to eat spinach with vinegar. Just steamed and with a splash of plain white vinegar. It is a very common way to eat spinach around here.
Anyway, I suppose it could have some affect on iron absorption. Large amounts of vitamin C can increase non-heme iron absorption by as much as 200%. This article, http://pubs.acs.org/doi/abs/10.1021/jf0203040, seems to indicate that acetic acid, the main component in vinegar, may also increase iron absorption.
Judging by the nutrition information I can easily find (for example, from nutritiondata.self.com), frozen spinach is generally cooked by boiling and draining. That's pretty much what I'd have guessed; it's certainly the easy way to cook things.
Unfortunately, that means that some nutrients are lost with the discarded water. I doubt it'd be different for any other green leafy vegetable - though it's certainly much harder to find frozen mustard greens!
The nutrition information for fresh spinach is for fresh, raw spinach; if you boiled and drained it, you'd make the same sacrifice that the frozen spinach has.
As for your generalization, freezing vegetables and fruits does preserve most things pretty well - but only what's actually left in them when they get frozen! And while transporting fresh vegetables around can cost you some "freshness", it's not going to affect minerals. Some vitamins could be lost by breaking down (I'm not an expert here, but it seems possible) since they're more complex molecules, but minerals are just single elements. Those iron atoms won't fall out of the spinach on the way, and they're certainly not going to be transmuted, either!
Edit: Essentially what's been said in the comments is that the USDA nutrition facts say that "Spinach, frozen, chopped or leaf, cooked, boiled, drained" has about half as much iron as "Spinach, cooked, boiled, drained". A discrepancy, indeed, though not the same one cited in the question. My interpretation here is that the frozen spinach is simply not cooked in the same way (boiled and drained more thoroughly) as the non-frozen.
An alternative, proposed by Adisak, is that the frozen spinach nutrition means that you've taken the already-cooked, frozen spinach and boiled it again and drained away even more nutrients. This seems unlikely; frozen spinach is already cooked, so there's no reason for the nutrition information to assume that you'll boil it over again. The description (it seems to me) is referring to the cooking that took place before it was frozen.
Best Answer
True, you may have cooked it too much and spinach is cooked when it reaches 160 degrees F. If the spinach overcooks, it does not necessarily mean it is bad, it is just overcooked. In my opinion, eat it, I have because mine did not taste bad as I understand yours did not taste bad.
Now, another idea is that when you have smelly spinach, you might have cooked in on very high heat and you burned it or overcooked it, you could still eat it. Again, if it does not taste bad, it is ok to eat it.
Another idea is that you used rancid oil, which using the rancid oil on the spinach would taste bad and you would then get rid of it. That would taste bad.
Also there is the problem of listeria, which is some kind of a bacteria, I think, and the spinach is bad to begin with and will or might make you sick. Please look that up, lettuce also has that problem. I think the spinach would have a blackish hue to it and taste bad. Get rid of it.
Lastly, what I read about spinach is that you do not re-heat cooked spinach, eat it cold. I make big batches of creamed spinach, my family loves it, takes it to their homes, nukes it, and nobody has gotten sick. I am no doctor, so take that info for what it is worth.
Lastly secondly, I use frozen spinach to make dips. I make too much of that also and I make my own creations of that. My family likes to take that home also. What I do with any dip is put out smaller portions at a time and fill up as necessary. It is time consuming but as a safety issue it makes me feel good and when you feel you are doing your best to keep people eating safer, it is worth it.
At very Last, use your judgement, don't take chances. When in doubt, throw it out. I do what I do because I have a lot of experience, but that does not mean I don't get into trouble once in awhile. Good Luck