My understanding of the single-word switch indicator is that it is to be used whenever a single word occurs between two math expressions. The only other option would be to go in and out of Nemeth around the word.
A colleague has interpreted the Guidance use of the word "may" in "when a single word standing alone occurs between two math expressions, a single-word switch indicator MAY be used ..." [emphasis added] to mean that instead of using the single-word switch indicator we can simply spell the word out uncontracted, if there are any contractions, and stay in Nemeth. I think that interpretation is ignoring the beginning of the sentence: "To avoid use of switch indicators ..." meaning open/close Nemeth.
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.
A possibly useful way of reading the "may" you asked about in the Guidance for Transcription Using the Nemeth Code within UEB Contexts is that it means we can either 1) use the single-word switch indicator or 2) close Nemeth Code before the word and then open Nemeth Code again after the word. Either way, the word is transcribed in UEB.