LizKay

I doubt they would check for plagiarism, they've got enough work as is. Just don't be egregious and you'll be fine.

Do not present others' work as your own.
For guidance, please see the following.
https://www.historians.org/jobs-and-professional-development/statements-standards-and-guidelines-of-the-discipline/statement-on-standards-of-professional-conduct#Plagiarism
http://www.oah.org/about/reports/reports-statements/professional-integrity-and-the-oah/
The position that plagiarism is "fine" as long as it is not "egregious" is not sustainable.
 

I think you're fine. Schools understand that typos in letters of recommendation aren't within your control.  

I was...checking my email?

Spelt my name wrong on my Stanford application....
 
Still got in!?

I use this rule of thumb: whenever someone gives of their time in service to me or my goals, I always send a thank-you email. This goes for small things, like emailing the grad assistant who squeezed me in to see a school administrator when he would be otherwise unavailable to the administrator himself after the meeting. Every favor, recommendation, meeting, interview, offer, and bit of consideration gets a (sometimes very quick!) email of thanks. I have NEVER irritated people with this practice, in fact, I believe it has helped make me some pretty strong allies over the years. 
I highly recommend everyone take up this practice! So many in higher ed are not thanked enough, those emails seem to mean a lot.

Colorado State: "Apparently you don't even have to apply in order to be rejected from certain schools. What a cluster F it must be over there that they're emailing people who have simply requested information that they're 'rejected'."

I'm with Hk328 on thinking that these deferments are for very special circumstances (and most likely relationships are not one of them, sry). Since this is something that they expect applicants to have in mind (family distance, relationships, etc), I don't believe a committee will give defer on such grounds, so ultimately it is up to you to decide.
There are some questions that you may ask yourself to find out where your feelings and motivation are (at least I considered these before making a decision):
-  if suddenly you find yourself without your partner after rejecting an offer (for any reason), do you think you would be happy with your decision?
- is the presence of your partner essential for you to succeed in your studies? usually, the first years are brutal and you don't have much time anyway
- do you think s/he would do the same for you?
- if you are rejected for asking a deferral, how do you think you would feel? 
- if you receive an offer and not get the deferral, decide to maintain your relationship and reject the offer, but next year you don't get an offer and your partner gets one, how do you think you would feel?
- if you don't apply (or enter) now, what are your plans to still be competitive for the next applications round?
- If ten years from now you are either 1) with this partner but without dream degree OR 2) with dream degree but without this partner, which one would you choose?
Don't need to answer here, those are just to help you to think about the situation.