I have seen few reports of folk being issued boarding pass with return flight past the (previously) 30 day visa exempt entry.
A signing of a waiver sometimes asked for by supervisor.
In any event from the OP......
"think maybe best if I go for the Non O. The thought of being knocked back (however unlikely) at Heathrow, wife in tears etc., is too much of a worry."
Seems that the OP is not interested in possibility of drama.
Perhaps for that reason he is leaning to Non O eVisa over visa exempt (that also requires extension at immigration)