You seem to ignore evidence pretty well.
$31, 19, 25, 20, 10!
To be honest, I didn't really look at the specifics of this sale. I'm just saying that in general, if you have a commodity or commodity-like item, which I believe standard Model M's are, and you have several dozens to choose from, offering to sell 1 or 5 at one time shouldn't have a significant impact on the selling price in and of itself, because the quantity doesn't affect the total quantity for sale by all vendors by a significant percent. Other factors may though have a significant impact. It was mentioned that the last one had missing keys and was in bad shape. I think that was likely the over-riding factor in the price being so low, not the fact that it was the last one sold. Even you, yourself commented, "One of them looks dirtier than the stove after I make the gravy."
As for the remaining ones, being I didn't look carefully at the listings, I don't if there was a significant difference in condition between them to account for the price differences. Anyways, throwing out the last one, there's only a $12 difference between the highest and lowest, and the lowest was the second one sold. Which doesn't do much for your theory that the prices should be declining, because the prices after that went up. But regardless, that $12 difference isn't a lot for many people. If you were talking SSKs selling for $310 and $190, then it's of much more significance. For small dollar items, and where shipping is a huge part of the cost, $12 isn't much to hang your statistical hat on in my opinion.
That sort of a price difference may also very well not be so significant to the seller. Perhaps the seller wanted to sell them all at once, deal one time with packing them up and shipping them, and be done with it. Having to pack multiple times, make multiple trips to get them shipped, etc. might not have been worth the time to potentially make a couple more dollars.
Anyways, I agree multiple simultaneous listings for similar commodity-like items can reduce the bidding by a small amount. I wouldn't say it has no effect. I'm only saying the difference is quite small and likely not a significant factor for many sellers. If you're entire business and livelihood is selling items on eBay, then you'd pay attention to stuff like that and try to get the maximum profit possible. For some guy who just decides to sell some stuff though, I don't see him jumping up and down, getting all excited over making an extra $12, or not making that $12 that he might have. That's really all I'm saying.