1. ON", "OFF", and "GAME OVER"
These are likely recognizable to most people, regardless of language. Still, they look a bit out of place when everything else is translated - almost as if they were accidentally missed. Don't you think it's worth translating them for consistency?
2. "JETPACK"
In many languages, when there's no direct equivalent, the word is transliterated. Would that be a suitable approach for Greek as well?
3. "TUNNEL1" and "TUNNEL2"
You are correct, these are not very important.
4. "TOUCH BUTTONS OFFSET"
5. Shop text
You don't have to stick to a literal translation if it doesn't work well in Greek. Reordering the lines is also fine if it reads better.
For example, it is possible to make something like that:
"'S SUPPLY SHOP!": "WELCOMES YOU IN HIS SUPPLY SHOP",
"WELCOME TO ": " ", # empty line, but must contain at least one space, otherwise the original line is used
Result: [SHOPKEEPER'S NAME] WELCOMES YOU IN HIS SUPPLY SHOP
6. Diacritics
If other diacritic symbols aren't used in the translation, there's no real need to add them - it won't hurt, but it also doesn't add much value.