Thanks for the reply, bostonphil!
Well first of all, you do not know what time your flight is really
going to arrive. Scheduled to arrive is one thing and actual arrival
is another.
You might not arrive in Mexico City until 9 pm.
True; but it's a short domestic flight from Mérida. The likelihood of a long delay is small. Weather won't be a factor.
Will you be just boarding or do you have to go through another security / immigration at Mexico City before your trip to Istanbul?
Two separate tickets. So yes, I will have to go through security. At 3am, the lines will be very short. I've done this early morning Turkish Airlines flight from MEX to IST before and zipped through security. And MEX doesn't have a passport control line to go through. As a permanent resident, I just have to go to a counter to get my US passport stamped. There's never a line to do that.
There's an outside chance I will already have my Turkish Airlines boarding pass on my phone, but usually they require me to check in at the counter.
What is the seating like in the Mexico City Airport?
Terrible. It's an awful airport to have to hang around in, both landside and airside. I'm flying Business Class on Turkish Airlines, so once I get through security, I can use the lounge, but that'll be after 3am or so.
I would just shower at home and bring a bunch of food with me and hang
out in the Mexico City Airport . You can people watch. Maybe
restaurants will still be open. Hopefully they have comfortable
seating. You will have your technology.
Yes, that's what I'd do if I go with option 3. You've done a nice job of encapsulating the debate I'm having inside my brain!