Zoeeeeeee

As long as her passport isn't expired the moment she enters the country on her K1 visa she should be fine. Just keep in mind it's a pain in the ### and usually a lot more expensive to get her passport renewed here instead of in her home country. 
 
She can renew her passport already before filing, or renew her passport before she has the K1 interview. It won't impact the interview. I would consider one of these options instead of entering with a passport that's only valid a few more days after she enters.

No need to delay filing the petition if you have all the requirements ready.  After filing, ask your fiancee to renew her passport before completing her DS-160, so she can enter her new passport details in the form.
 
Also, what country is she from?  In my country, people are usually not allowed to fly out with a passport that's valid for less than 6 months.  Checking is done by airlines upon check-in or by exit immigration officers.  This is because of the difficulty in renewing the passport when abroad.  Might be good to check if your fiancee's country has similar rules.
 

Come on @LukeU, I’m not going to be able to sleep until I know you’ve got a March date booked! 😳😆

Come on @LukeU, I’m not going to be able to sleep until I know you’ve got a March date booked! 😳😆

I just want to point out to all the K1s here that you are on a spouse visa and can do that. You enter and become a permanent resident that day so are free to leave to go sort your things however you like.
 
The K1s can not leave until they marry, apply for AOS/EAD/AP, and receive approval for their Advance Parole travel document. 

Yes that's a fair point. Was looking at march 12th anyway, can't find flights earlier than that (BA reward flights).

I don't know if it's a bit silly, but I was going to fly over to get the PR stamp in my passport, find a flat to rent, sign the lease, apply for an SSN, and then come back.
Then wait another month or so before we move out properly.

Come on @LukeU, I’m not going to be able to sleep until I know you’ve got a March date booked! 😳😆

Wake up @LukeU !! Where are you? March is open and you haven't said a peep. 🤐😂

I just booked a march date too there's lots open now

March appointments are all open!! We have ours scheduled for March 5th.

Yes, they will return the passport upon request. It is not their property. It is not even your own property for citizens of most countries. It is generally the property of the country that issues it (hence why a country can revoke one of their citizen's passports).

I think I should be ok - I mean, surely if you’ve started going through the K1 process, that would be the time you’d be least likely to illegally stay...? I’ll have a return ticket booked and I’ve been many times and never overstayed, so I don’t anticipate problems...country of origin I’m sure plays a part too - coming from the UK, I assume the expectation of illegal immigration is reduced...

I think I should be ok - I mean, surely if you’ve started going through the K1 process, that would be the time you’d be least likely to illegally stay...? I’ll have a return ticket booked and I’ve been many times and never overstayed, so I don’t anticipate problems...country of origin I’m sure plays a part too - coming from the UK, I assume the expectation of illegal immigration is reduced...

My husband is from a similar country to UK (NZ), and had no issues traveling here to the US on his B2 with our K-1 pending.  In fact, he was here when we received our NOA2.  He had a return ticket and evidence of loose ties to return to in NZ, such as a house rental agreement etc.

My now-husband did this twice. No issue.

We went through a similar situation. My fiancé had his interview and they kept his passport but they requested additional documentation. While he was working on the additional documents, he had to travel back to his home country to attend his father's funeral. They returned his passport to him and asked for him to return it to them when he got back to his residence country where he interviewed. So it is possible, but I don't know if it was based on the country or the extenuating circumstance of a death in the family.

I don’t have any advice to offer but I just want to say I am so happy you are keeping your dog and making these crazy travel sacrifices for him/her. So many people would just give up their animal. I have a cat with a heart issue and although she doesn’t travel I understand some sacrifices that have to be made for them. Good luck on your visa journey  

while this is Ok in UK to vJ members reading this and you are from high fraud country,  it is not the same
this is so country only applicable
 
in high fraud countries which is most  of the world
https://mypathtocitizenship.com/which-countries-are-on-the-uscis-high-fraud-lis
once your passport is asked for,  you will not get it back till the visa is ready or denied
this is so those in high fraud countries can  not travel abroad after interview as he/she could travel to a country the US deems a terrorist threat and all this requires  additional security checking
that is why we saw the post about a month ago   from a Senegal man (who interviewed in Morocco)and has been in AP  for almost a year and they have kept his  passport all this time

They don't notify you. When you get back, you notify the courier to pick the passport up from you to take to the embassy. It's the same DX courier that is contracted to deliver passports/visas. Then allow several days for somebody to pick up the case to work on and pull your file. Then you are back to the "7-10 days" or "10-14 days"...whatever they are currently telling people to allow for processing. So if it all goes well, maybe minimum of two weeks.
 
But from what you wrote, you plan to interview after your August trip so will be leaving your passport with them on interview day. 

Your visa will expire 6 months from the day of the medical so you might want to push your medical closer to your interview so you don't get a visa good for 1 month or less...just in case you get delayed.
 

Yes, you can request to keep your passport. They just won't be able to issue your visa since they won't have your passport to physically affix it to. They will notify you that they require your passport back, and you can then return it to them at that point. It's a fairly common request, particularly for people who travel often.

Don't do it (pretty rich coming from me). Just check your vaccines, get what you can off of the NHS. You won't require a flu shot if you're aiming for May/June time so that's one off the list already! 

Oh hell no, I’d much rather have details about everything, there’s no such thing as ranting too much when it comes to this process (in my opinion). Expecting to do my medical in May-June, so plenty of worrying-time ahead of me! 😫

I maybe ranted on a little too much, but I was so thankful to read reviews before I went for my appointment. 
Like you say, you're still going to think the what ifs, worry for the worst outcome like I did for weeks on end. But it really is just like a check over for when you go to a new doctors 🙂 
 
I hope yours goes as smoothly! Here's to hoping after we leave there we don't hear from them again! 😄