In your position, or if I found myself in identical circumstances, I would not be able to resist engineering a way to entice some conversation about shoes and heels. It's just too much of a temptation....
Strategically, you could do with an ally or allies. Seems you might have them lined up anyway? Both your son (who seems like someone you are doubtless very proud of) and his girlfriend are two people who can be trusted. If Mrs R-I-B could be induced to helping, perhaps she could ask the girlfriend discreetly and informally, what she thinks of a man wearing (essentially) girls shoes by way of a girl-to-girl chat? Of course this notion might be completely unrealistic (I don't know your circumstances), but you are steadily moving toward a fairly open position about your heels, and I would think the longer you can have some control over the situation (as little as that might be) the better for you if a 'damage limitation' position is necessary. That's when having some additional family support might be helpful in maintaining some balance to a situation that could all too easily turn sour. I suppose the overall thinking is that bringing anyone 'onboard' with your shoe wearing, is going easier with greater chance of success doing it one household at a time, rather than having to confront a larger group who might already have made up their minds. Though I also feel you would/will have less of a battle than most of us. Given what you have revealed about how accommodating Mrs R-I-B has been, and how some of your immediate family might be just as supportive, I'm hoping to hear some good news in the not-too-distant future.
On a side note, I was very pleased to read about the 14 year old who is getting support for her emotional needs, where her parents are not so obliging. We lost a family friend who had the same 'problem' and who didn't find the confusion easy to deal with. The truly upsetting thing is that having decided to go the journey and transition - so seemingly happy about the way forward - a night of bingeing on alcohol brought about a set of unfortunate circumstances and the loss of her life. Unlike the 14 year old mention, the family friend had support from everyone around her, but years of being confused about how she felt and why she felt, had taken its toll. No-one thinks self-harm was intentional, but a life-style created as a coping mechanism, may have gotten out of control. The conclusion I reached from this, was that it isn't possible to be overly-careful in these situations, especially with children who seem to be susceptible to giving in to dark thoughts simply because their age makes them prone to emotional extremes. As we read more and more every week, it appears there is a lot more 'gender middle ground' that had previously been realised.