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 And anything that happened at the speed of light would have too many unknowables attached to begin with—closer to religion than science.

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"closer to religion than science"
This sounds like some of that "common sense" pablum. You know, when some dolt says, "He may be smart but he doesn't have any common sense," to make them feel superior to the smart person. But common sense is stupid. It's observation without scientific backing! It's making assumptions about the world based on basic perceptions! Common sense says that the sun revolves around the Earth. Because you can see the sun moving across the sky. That's common sense. Science is all, "Yes, that is what it looks like! Good perception! But I'm about to blow your mind, baby!" "Common sense" is closer to religion than science ever could be!
    People also love to point out that people who believe in science have faith just like a religious person. They have faith in their theories and whatnot. But obviously that's about as insipid an opinion as somebody could vocalize! Scientific theorems are based on evidence while the main attribute of faith is that it cannot have any evidence to support it. If there's evidence to support a conclusion, there is no room for faith. So shut up with your faith-based arguments against science, stupid people!

Although, really, I get what Merle is getting at. Science always seems to have an event horizon which we can't imagine ever truly being able to understand. In the 1880s, the speed of light must have been bewildering! But they had already developed experiments to begin working with light and methods with which to measure it and observe how it might change based on other variables (the whole point of the Michelson-Morley experiment). I think about this limit in terms of The Big Bang. I can't imagine how it could ever be knowable what was happening on the other side. I don't see theorizing about it as being close to religion though. But then I've been areligious my entire life and so I'm not too fazed by unknowables. Let them be unknowable! I've got turnips to buy!

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 Exists, doesn't exist, what's it got to do with the price of turnips basically.

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"Exists, doesn't exist"
Whether something exists or doesn't exist would seem really important when you get right down to it. But Merle acts rather casually about it and I get it. I once dropped out of a philosophy class in college because my attitude was pretty much this. It felt like everybody in class just believed philosophy was throwing up a new roadblock to anybody else's suggestion on whatever subject was being discussed. It was fucking tiresome. I was just kind of like, "Fine, y'all argue about whether or not what we see is real. I'm going to go get laid. Maybe that won't be real but, man, I'm going to enjoy the hell out of whatever it is!"

"price of turnips"
I guess at some point in Irish history, turnips were so important that if you tried to talk about anything else, people would say, "The head on you and the price of turnips," meaning, well, I don't fucking know what it means. But I guess the price of turnips (and the price of cabbages) was meant to represent some fact that actually matters to the lives of everyday people who don't have the time or luxury to mess with all of this universal hoodoo about natural physical laws and whatnot. So Merle is just pointing out that whether or not Æther exists doesn't mean much to the lives of most people.

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 He had already heard in some dim way about the Æther, though being more on the practical side of things, he couldn't see much use for it.

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"being more on the practical side of things"
At this point in Merle's life, when he's living in Connecticut, he's building machines for Heino Vanderjuice. Later he'll take up an interest in photography, possibly because he's interested in the physical aspects of the machine and how it works at first but later, he becomes quite invested in how many boobies he can see using it. What I mean to say is he's a man of the physical and of what can be seen. Theoretical concepts may as well be magical chants or religious rites. Maybe! I'm projecting and assuming here, I'm sure.

"he couldn't see much use for it"
I think this is a joke! At the time, Æther was supposedly what allowed light to get from one place to another. And without light, you can't see. So not being able to see much use for the thing that enables you to see would, to my mind, be a pretty good joke! Good one, Pynchon!