- Needles through TSA
TSA agent: Wait, sir. What are these for?
Dexter: Diabetes. I have a prescription if you wanna see it.
TSA agent: Go ahead.
Unused syringes are allowed when accompanied by injectable medication. You must declare these items to security officers at the checkpoint for inspection. We recommend, but do not require, that your medications be labeled to facilitate the security process.
- Exiting (WITHOUT a body) after checking in and going through TSA
I can't find definitive information on MIA (Miami International Airport) but generally at US airports, you can always leave the airport unquestioned even after checking in and going through TSA (provided you are not leaving with a body on a stretcher).
- Viktor fails to get on plane after checking in at the airport and going through TSA security.
Airport security camera footage will have Viktor walking into a toilet, Dexter following him in, Dexter injecting him with something in the toilet, Dexter wheeling him out on a wheelchair unconscious, etc.
Viktor is later a suspect---see e.g. S07E05:
Angel Batista: Deb, I still think Viktor Baskov killed Mike Anderson and Viktor just disappeared into thin air.
Angel is a sufficiently competent investigator who would have easily found the above airport camera footage.
- Spending significant time (at least an hour?) undisturbed at "Okhotsk Air Unclaimed Baggage" room setting up, killing, and cleaning up
Again, even if Dexter is sufficiently lucky to be undisturbed this entire time, all of the above will be captured with cameras.
- Exiting the airport WITH a body on a stretcher
The only images we are shown after the above kill room images are of Dexter at an airport car park transferring the body to his car (again, all would be captured on camera):
We are not shown how Dexter gets out of the airport with a body on a stretcher. It is not plausible that Dexter managed to do this.