So yes, the code isn't going to stick using the RTE. Yes you can use the RTE, go to Source, insert the code, and save the page; if you then open up the story viewer and go to the page in question, you'll see your text centered just the way you want it.
But as soon as you go back into the RTE and start editing that page again, your extra code is immediately forgotten. Therefore placing the code would have to be the very last thing you do.
The RTE was upgraded a few months ago, and I believe the omission of left/right/center paragraph options is a bug that can be fixed.
Those grayed-out indenting buttons aren't what you think they are. You would use them if you were creating an outline, like this
- Main Topic
- Indented sub-topic
- Indented sub-sub-topic
- Less indented sub-topic
- Next main topic
Those buttons aren't for paragraph indentation.
If you're relying on a lot of codes for special formatting, then you might keep your sanity more by turning off the RTE.