Capitalise at the start of sentence IMO. If in doubt use grammarly.com and set it to your desired English variant. It’ll give basic hints on grammar for free. I have it set to British English and use the plugins for firefox/chrome/Vivaldi and Outlook/Word.
For the purposes of your question, the first two with the commas is correct, but it depends on context. The second set are more like questions than commands. “Remember John” is asking a third party to remember John, not asking John to remember something. “Come John” doesn’t really make any sense, though. It ought to be “Is John coming?” if it was meant to be a question to a third party. A reader might infer that from the question above it. Possibly. Unless… it was part of a bedroom scene, then it might make sense. I’ll assume that is not the case, though, since it would still be wrong.
A good way to remember commas and how to place them is that they are meant to be natural breaks in a sentence; those moments that, when reading a script aloud, someone would naturally break or change tone. That is how people know where the breaks and changes of tone are in a script. It’s advisable to read your work aloud and see if it feels right to say. Narrate it, effectively. If it doesn’t feel natural to speak, something is wrong somewhere, usually wherever it starts getting uncomfortable to read aloud. Language is meant to flow effortlessly, and not to be a chore.
Anyway, that’s enough of a lecture for one day, I think.