An interesting and thought provoking post Nell which I thought I'd respond too as it makes for an interesting debate.
Does M "threaten our reserve and therefore our intrinsic Britishness"? I'm not convinced there's such a thing as intrinsic Britishness but do accept that there is for some.
To infiltrate is to enter or gain access surreptitiously and gradually. I wouldn't describe M's entry to the RF as an infiltration, there was no subtlety that I could see, and rather than being 'a breath of fresh air' as I think some regarded her, IMO she was more like a tornado.
Whether or not it was her intention to 'break' the monarchy is known only to her, but if that were her ultimate goal she has so far failed to do so and I can't see her ever succeeding as they are increasingly distancing them selves, not just from the institution of the RF, but from those personal family relationships.
Your reference to making herself equal was interesting and made me think about what that would mean. Would it mean being equal with the other family members? If so in terms of her personal relationships I'd have thought that was a given. Even with the Queen who after all is her husband's GM.
In terms of her standing within the court and creating a level playing field then no. Harry is 6th in line to the throne so extremely unlikely to ever be king and in so becoming, make M his queen.
I've often wondered how much of M's 'unhappiness' stems from the knowledge than one day, in public at least, she would be required to show a degree of deference to her b.i.l. and s.i.l. when they become King and Queen.
I think M ever being in a position to be a catalyst and figure head for deep change within the RF is unlikely. That said, by living in the UK and having a working role she may well have been influential in some areas,
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As it is, that opportunity has been thrown away and I can't help thinking that when she married H, in terms of bringing about change within the RF and possibly elevating her own position, her reach extended her grasp.