Thank you for the question and the supporting documents. Your concern is understandable.
First, I agree with you wholeheartedly that the "and"/"y" as well as the "or"/"ó" are part of the mathematical expression and not part of the narrative text. So, for that reason, the single-word switch indicator is inappropriate.
Second, I would also not use the single-word switch indicator for the Spanish symbol for an accented o, because the single-word switch indicator signals that what follows is a UEB symbol, and the one-cell symbol for the accented o is a Spanish Braille Code symbol.
Third, if the Spanish Braille Code symbols "y" & "ó" are transcribed within Nemeth Code switch indicators without any additional indicators, then they technically only have the meaning of their respective dot configurations in the Nemeth Code. Thus, the "y" is the 25th letter of the English alphabet, and, more troublingly (as you say), the Spanish Braille Code symbol "ó" is a plus sign.
We are left trying to use Spanish Braille Code symbols within a Nemeth Code context, and that is a no-win situation.
So, since we've got to do something, I would go with your first suggestion, under "Spanish uncontracted," because it is probably the least likely to be misread. Thus, we get:
,si ,a y ,b son sucesos traslapados1
entonces _% ,p(,a + ;,b)
.k ,p(,a)+,p(,b)-,p(,a y ;,b) _:4
Please note that the ELI is not used with the As that are only in contact with one grouping symbol, because if that opening paren were not there, then the A would be unspaced from the P. This is in line with Nemeth Code section 28.a's directive that when one letter is in direct contact with only its opening or only its closing grouping sign, the English letter indicator must be used or must not be used as though the grouping sign were not present.
Regardless of how you handle this Nemeth-Spanish conundrum, a transcriber's note is in order, I think. Perhaps something like: Spanish language words "y" as well as "ó" are used within the following equation.
I hope that either my thought process or my suggestion are helpful to you. Thank you again for the question.