applicant2

@applicant2 just to be clear, your passport needs to be valid for most international travel, especially by airplane. The extension letter is usually either 18 or 24 months. It extends the validity of your expired conditional green card by 18 or 24 months. That's 18 or 24 months from the expiration date of your conditional green card.
You should have valid passport, expired conditional green card and valid extension letter for travel.
 
To your question, no, unlike passport you can use extension letter until very end, e.g. enter on the day it expires. But it's best if you're back a few weeks before its expiry to avoid any unexpected delays.

3. 90 days prior to the 2nd year anniversary of your conditional green card. For example, I was a resident since 04.29.2020. Two years later =04.29.2022 -90 days: earliest filing date for me was 01.29.2022

1.  2-year GC can be used to re-enter the US until the expiry date listed on the card, or the extension date based on the USCIS notice you will get when removing conditions.
2.  I-751
3.  Check here -- https://www.uscis.gov/forms/when-to-file-form-i-751
 

A k1 is not a v series visa.
 
If you filed the AOS prior to the medical expiring then you do not need a new medical as long as it is marked complete.