British native, not a teacher.
Firstly it is spelt grammar!
1) Two week intervals could be understood as two intervals of one week, so to be clear, you should hyphenate the words: Two-week intervals. You don't need the plural s on week.
2) Two weeks old is correct. For a baby that is two weeks old, you would say it is a two-week old baby. "Two week old babies" would probably be understood as "two babies, each one week old".
3) I would say 'An': An hotel, An MPV full of children, an HPV vaccine…
4) If I were using the term RBC to mean red blood cells (plural), then I would probably make this clear by saying "RBCs carry oxygen" The reader might know that RBC means red blood cell, but they wouldn't be able to assume that the abbreviation was already plural - so add the s.
Hope that helps.
Student or Learner