MacPara

Yes, it's a difficult system to navigate. I was on a J-1 once, so I remember the struggle. The "you can't leave once you file" statement only applies for AoS. The alternative is that you maintain the timeline you laid out in your original post but then you stay in the UK after visiting your family for Christmas and file for the GC through the consulate. That would prohibit you to reenter the US on a non-immigrant via like your J-1 as your GC application has now shown your intent to immigrate. I strongly suggest you heed the advice from the other folks who commented here and have your family visit you in the US for Christmas instead, if you cannot do the paperwork right now. 

@katywalla read my comment posted earlier.
 
You're OK marrying now, filing for adjustment, and leaving the US for trip in December if you have advance parole or green card then.
 
Why is it OK to adjust now? Because you didn't have intent of staying in the US when you last entered to the country.
 
However, if you leave now without filing for adjustment and plan to reenter with intent to adjust, then you have potential issue with misrepresentation / fraud in the future. 
 
Most of AOS applicants have interviews nowadays. At the interview adjudicator may dig deep into your relationship timeline, reasons for marriage etc. If they suspect you played the system by entering the US with immigrant intent on non-immigrant visa, you'll have serious issues.
 
Also, the chance of reentering on J-1 after marrying US citizen is also decreasing significantly. That's why it would only be safe to leave on Advance Parole, because then you would be upfront to CBP: yes, I married, yes I have immigrant intent, here's Advance Parole.
 

Yes, legal immigration is difficult.  In our case, my wife and I (already married) were separated for a year during the process before she immigrated to the US.  

Illegal route may be even worse :
1) OP marries in October
2) Leaves the US
3) Attempts to reenter the US on J-1 with intent to adjust
4) Gets caught by CBP. Held in secondary, questioned, phone and personal belongings searched
5) J-1 is cancelled, entry denied, sent back home
6) Now OP is facing 3+ years consular process overseas AND needs to seek waiver for misrepresentation

90 days is irrelevant.  A person cannot enter the US as a non-immigrant with the intent to stay and adjust status.  That is visa fraud, and it is abuse of the purpose of a non-immigrant visa.  Immigrant visas are intended for immigrants.  Non-immigrant visas are intended for visitors and non-immigrants.   This is not the time to try to get around immigration law, imo.

No.  It is illegal to enter the US as a non-immigrant with the intent to adjust status.  That is visa fraud.

Yes.  In Dallas, it took 3 months to be notified of a future interview in 2022.

MyProgress estimate can be safely ignored. Some people saw "7 months" until decision on day of their oath. Others waited way longer than initially shown 5-6 months estimate.
 
Also, the visual part is very confusing. Current step doesn't mean it's completed, it means it's the next thing to happen. Especially if it's an empty circle, not filled with blue and checked.