If I were in divisi like that i'd take it as implied, especially if the texture of the music doesn't lend itself to half a section just dropping out like that. It really can't hurt to be abundantly clear and put unis. but if you feel it's cluttered I'd take it out.
Save a question being asked. Add in the text unis. as you have above.
Questions take up time, so just add it in. It's expected and not clutter.
That way was my first thought too.This appears to be asked and answered, but another option, if you want to avoid writing unison and then going back to divisi, is to simply show the unison as a divisi like so:
I would write it this way instead of cluttering up the score with extra text, but that's just my preference.