Probably not. The question is, what does the prepositional phrase modify: "largest" or "built." You took it to modify "built." There's no syntactical requirement that it modify one or the other, that I'm aware of. The choice must be based on meaning and interpretation. Few, if any, can take over a century to build anything, so modifying "largest" is more reasonable.

Prior to this, I had no clue whatsoever about wet plate photography, I did not know it existed, and I did not know the name.

So to me the headline was very confusing - when I hear plate, I think of tableware first. The association I had was like an art exhibit of wet tableware.

I have to admit I am a little disappointed that the actual content of the article was far less weird than I had expected.

I think that would be an odd way to read it. It's an idiom, I guess.

"It is the most significant archaeological find in 50 years" = It is the most significant archaeological find among archaeological finds of the last 50 years

I think you might be. The "in" at the end makes it pretty unambiguous.

Maybe. I dont read the title how you do.