There is an obvious narrative intention here, given dialogue.

Redbelly is supposed to be nothing but an iron mine. Been working it for years. Then right before the spiders had moved in, we found that chunk of ore. Never seen anything like it. I want to know what I'm dealing with before I start tearing it out of the ground.

Sorry, the mine's dry right now.

According to the dialogue, Redbelly is very clearly a former iron mine. Changing the ore inside to iron would be to ignore this dialogue. Grogmar buys iron because that's what he's used to, and that's what the whole operation had been about until very recently. He doesn't care for ebony, he doesn't know what it is, he probably doesn't even have the tools to work it. I changed the ebony chunks outside to iron, to represent this - the last bits of ore they had managed to scrounge up before being forced out of the mine, and a clear indication of the sorry state of the town's economy - just three pieces left and it's all gone.

ESO (though I don't personally consider it relevant, this is just an aside) has Redbelly Mine as an ebony mine. This might have been the origin of the town's name, Shor's Stone, as ebony is considered the crystallized blood of Lorkhan/Shor. Like many real-world toponyms, the name with its meaning remains, even though it may no longer represent the facts of that location - take the town of Oxford, ostensibly once a location where oxen could ford a river, now a university town.

In the case of Redbelly Mine, I therefore consider it a mixed mine, where ebony had been mined in the past (giving the town its name), and dried up. Long by the time of Skyrim, iron had been discovered and ebony fell out of local memory, before those deposits dried up too, and ebony was revealed once more, shortly before the player's arrival. Northwind Mine, by contrast, is most likely connected to the same underground iron complex that Redbelly Mine once exploited, which is why it is an iron mine in the vanilla game and not an ebony mine (and there is nothing in-game, dialogue or otherwise, to suggest Northwind should be anything but a current iron mine). On top of that, iron and ebony in the same general location, or structured in layers, is not unprecendented: Gloombound Mine contains a few iron veins alongside the rich ebony deposits.

Regarding Quicksilver:
The only reference to quicksilver being in the picture at all is the nature of the single ore sample Filnjar gives you. I will therefore consider this to be a true developer oversight, given that in the vanilla game there are 3 ebony samples outside and 3 ebony veins inside at the very bottom of the mine, apparently newly discovered. Given the apparently deliberate reality of the mine in the vanilla game, and the dialogue about Redbelly originally being an iron mine, it is crystal clear that ebony is the unusual ore discovered, not quicksilver. In any case, I give more weight to three ebony veins and three ebony pieces over a single quicksilver sample in my decision, because that's 6 developer actions versus 1 - more likely that 6 in a row were deliberate.

The only alternative solution to the Redbelly Mine issue is to do the opposite; keep the ore sample as quicksilver and change the ores at the bottom of the mine to quicksilver, at which point the debate becomes more about balancing. USSEP didn't do this either, so its changes are frivolous & wrong no matter what. Still, given all the above, ebony seems more likely.

The 3 ebony samples outside in the vanilla game are a little bit more complicated. Filnjar wants to know what he's dealing with before he tears it out of the ground, yet there are already three chunks torn out of the ground. Perhaps the other miners did it, perhaps they form the rest of a larger initial sample that Filnjar gives you a piece of, perhaps it was an oversight. I dealt with this by changing it to iron, as already discussed, for consistency with the fact of Redbelly being a former iron mine.