Have you read other alternate histories to see how yours is different? Such as: FATHERLAND, THE MAN IN THE HIGH CASTLE, THE ULTIMATE SOLUTION, CLASH OF EAGLES, and especially IN THE PRESENCE OF MINE ENEMIES, which takes place in 2010 in Nazi-occupied America and is about a 10-year-old girl who discovers she is Jewish.
Now for the query–
1st paragraph: I would refer to this as Alternate History rather than just Spec Fiction.
2nd paragraph: This is written well and gives me a good feel for the story. One minor point: I would say Spider tattoo on their arms, to rid any chance of it being a living spider.
3rd paragraph: This is all infromation that an agent doesn’t want or need to see. You are almost justifying your book and defending your premise even before the agent reads the pages. This information might come up after they request a full.
4th paragraph: I would tone this down. Drop “sincerely”. Just say, Thank you for your time and consideration. ALWAYS put your book title in caps. This should say “per your request” or “as per guidelines” not “per to request”.
Question: They really wanted an attachment rather than pasting in the body of the email?
Drop the thank you again sentence, because this is laying it on too thick.
As always, this is just my opinion.
Good luck with your endeavour.