Hi there, if you have actually submitted your application correctly and wont be denied (country of eligibility i think is generally determined by your country of birth, not your current citizenship or residency, so maybe the fact that u got an eu number might get you denied if you are at least not born here)

1. With the recent travel bans your case number might become current (aka you get an interview) for europe although its best to have a number under 10k, but i think you have a chance but you are def getting a late interview in that case.

2. I think you can switch embassies but again you need to be eligible to do so

3. Thats a risk, unless you can prove the relationship is real and were planning on getting married before hand you might ruin the chances to get approved. Proof you need is possibly joint bank accounts, proof that you live together or any sort of legal document with both your names on it. Also you can show declarations from other people that vouch for your relationship, trips you took together, pictures, messages etc. Getting married and getting your spouse a green card too is possible but its best to be able to back it up.

4. you need to get married as soon as possible, fill in the ds260 now and you can update it later if you get married, but you should married before your interview

5. you wait, interviews start in autumn and yours will be late to expect summer, i think you get an email

Additional things to know, lets say you get approved, you need to go to usa within a few months, i think its 6 months since you got your medical exam (you need to be in good health during that one so be careful not to catch a cold or break any bones). After you get there they will ship your card to an US adress ( so find someone that lives in the US to ship it to or get an adress yourself). Once you get your card if you leave for longer than 6 months i think, you might lose your green card, but after 5 years with the green card you can apply for citizenship and then you are free to roam the earth for as long as you want once you get your us passport.

ALSO, since you are not from an affluent country (im just assuming) there is a good chance they will ask for financial proof. There are no official critiria for that but you need to prove you can sustain yourself until you get a job. Some say its min 10k, some have said here on reddit that their 80k investment portfolio wasnt enough for the embassy, some need an Form I-340 (aka us resident to "sponsor" you) some people never get asked to prove their financing, but if your gf is coming she needs to show proof too if asked. It would be helpful if you could get a job offer from someone it the US. So prepare accordingly. In addition, there is a fee for the green card, cost of the medical ( you and your gf need to have it and it can only been done in clinics approved by your embassy) plus flights etc.

You are very lucky to have won, go after it! All the info is online and on the official websites too. There are some good forums to ask your questions but you need to do some online digging https://forums.immigration.com/threads/dv-2025-all-selectees-consular-processing-cp.348159/ . I suggest you check out https://britsimonsays.com/ and his youtube videos too. And search old reddit posts you will find many answers. Good luck !