Yasna wrote:近所の家のおばさん means a "lady living in the neighborhood". There's no family relationship here.
Good to know.
In this case 見せてくれた means "showed me", so the lady showed the narrator a shrimp.
I'm assuming that was a mistake on my part as far as looking up things in my dictionary, as I learned later on when I found 思いこんでしまった in it. I should have taken another pass at some of these terms.
This part means "Thinking back now, the red swamp crawfish she showed me looked like a small shrimp because it was still a child"
So 見せてもらった means "showed me", similar to 見せてくれた above? If so, is there an difference between the two terms and their usage?
I suppose I translated the beginning of the English sentence a little more literally then you did. They have similar meanings. Though I might have placed "fact" in the wrong place. Iḿ still getting the hang of word order.
As below, I forgot to translate "like".
I also forgot about the meaning of から as "because."
"A red swamp crawfish is like a small shrimp"
See above.
You probably understand now, but the crux of this paragraph is that the narrator mistakenly identified the crawfish as a shrimp back then because it was a young, small crawfish.
Yes. I probably should have caught on a little sooner, but I get it now.

You could say "According to the lady, that shrimp was a creature called a "red swamp crawfish"".
That would definitely work. Though I wonder, would it be necessary to translate it into English? It seems kind of redundant to me. Or at least extra lengthy/specific.
そしてどうしてもそれがほしくなった means "And then I had to have it". That rule about ほしい not being used for a speaker's wishes only applies when it is tacked on to a verb.
I see. My dictionary only gives the rule, not the situations where the rule is in effect.
Thanks for all the help Yasna!
Seeing as my dictionary was at least partially to blame I wonder if anyone can give me any good recommendations for physical dictionaries?
I currently have the Kodansha Furigana dictionary. Which is good, but obviously problematic in places.
I do plan to pick up the Dictionary of Basic Grammar (or whatever the exact title is) by the Japan Times if I remember correctly.