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1. In this case, may I still mail in I-131 together with I-485 and I-864?

Yes. And also I-765 for free if you file this month.

2. If yes to Q1, as both I-485 and I-131 are required to do medical exam through I-693, $85 should I pay with which form? or twice? 

I-131 does not require a medical. If you file this month (March 2024), the I-485 fee, including $85 for biometrics, is $1225. I-765 and I-131 will be free if filed this month. If you file next month (April 2024 or later), the I-485 fee will be $1440, and I-765 will be $260 and I-131 will be $630.

3. I-485 allows and recommends to file I-693 later even wait till the inteview to bring it in inperson.

"You are NOT required to submit Form I-693 at the time you file your adjustment application, but may do so if you wish. Because of the time-limited validity of Form I-693, you may choose to submit your Form I-693 after you file your Form I-485. You may also submit Form I-693 in person at an interview in a USCIS field office, if an interview is required. By waiting to submit Form I-693, you may avoid having to repeat the immigration medical examination."

Correct.

But I-131 says 

" After you have filed this application, USCIS will notify you in writing of the time and location for your biometrics services appointment. Failure to appear to be fingerprinted or for other biometrics services may result in a denial of your application."

This is about biometrics. It has nothing to do with the medical.

I have already booked a medical exam apt on 4/1/24, should I cancel it and wait till USCIS inform us when and where to do med exam? Or we can still go on with our apt? Should we bring in an I-693 form? 

You can do either. You should bring an I-693 to your civil surgeon when you do your medical.

4. after med exam, should I mail I-693 in for I-131 or should we hold on it till the interview for I-485? or we'd better to have two copies? Could we?

Again, I-131 does not need I-693.