Hi Andrew,
To answer your main question 'When should we create a new profile'?...that's tricky. We have guidelines for IF a new profile can be created, but as far as I know nothing saying when a profile MUST be created. It mostly operates on a case by case basis, as far as I'm aware, and what people are saying is that in this case a profile should be created.
I understand your hesitancy to create the profile. Margaret does meet the basic requirements to have a profile created, but your concern is you don't want to make the profile right away while you're still unsure about the LNAB and other details of her life (correct me if I'm wrong on that point).
While I do not think a profile MUST be created, what I think and what others have been saying is that in this particular case it would be very beneficial to create it. If Margaret had a profile, that would be a great place to collate current research on her LNAB. It would make it easier for other researchers to find and contribute to, rather than just having her in the notes of someone else's profile. This would definitely aid in the LNAB search, to my mind.
I appreciate that you only want to add the profile once you are assured of the quality of the information contained within it. I understand all too well the tendency to only want to add a profile once we have all the information on a person, but the great thing about WikiTree is things can always be edited, changed, and updated as we go along. You can add Margaret More as a spouse of Edward, with an 'unknown' LNAB for now, and a death date of 'after 1558'. This is not invention or rushed guesswork, we know these things to be true, even if the information is limited. You can go back later and amend/add information as it is found. This is the approach I have seen most often for deep ancestry profiles where we only know the first name and rough life span of a person, which is the case for many early modern and medieval women.
A lot of profiles here are works in progress, and that is just the nature of this platform. We all work in different ways, but there is certainly a very good case to be made for creating the profile, as has been outlined in some other comments on this post.
~ Oscar