You are correct. A single-word switch indicator is required.
Most readers prefer the contracted forms of words when reading braille - which is one purpose of the single-word switch indicator...to allow the text to switch codes without having to use the open/close indicators. In the phrase "2+3 and 4+6" the word 'and' should have a switch indicator and should be contracted. The other purpose of the single-word switch is to let the reader know that the word is not part of the math. That's the reason you would need a single-word switch indicator on a word between expressions even if that word doesn't use a braille contraction.