No, you can't say "could be enter". It doesn't mean something different, it's just wrong. This is because the sentences are referring to the present continuous tense, which is made with the verb "to be" and the present participle of the verb:
- France is entering a new age
- jihadists are entering Tunisia
- she was eating spaghetti
- they were sleeping
What might make the original sentences more confusing are the use of the modal could. When you use could, the verb following it is in the bare infinitive, but if you're using the present continuous, the participle stays a participle. So it works like this:
- present tense: She eats spaghetti.
- present continuous: She is eating spaghetti.
- modal could and present tense: She could eat spaghetti.
- present continuous with modal could: She could be eating spaghetti.