Si Polr is impossible, as you say.

So what would vulnerable Si look like, in your understanding?

I just can't imagine an ENFJ fading into the background

What is the justification for this assertion?

the personal, subjective nature of Si

My understanding of Si is that it is personal/subjective in terms of interpreting sensory stimulus and experiences personally. So instead of liking or disliking strawberries based exclusively on how they taste, it's liking or disliking them based, in part, on the fact that you used to pick strawberries with your mother and it reminds you of happy memories.

the situational aspect of the auxiliary function and seen as a an enjoyable break from the dominant attitude.

This is not how I understand the auxiliary function at all, and actually I've never heard this explanation before. I'm fond of John Beebe's "parent function" explanation:

Your auxiliary function acts as the “good parent” to your dominant function (or Hero). It will be the opposite orientation, meaning that if you extrovert your dominant function, your auxiliary function will be introverted. It’s nicknamed the good parent, because it’s supportive, we use it when we help ourselves, others and we use it to complement ourselves. This is Alfred the Butler, always there to pull Mr. Wayne back up, trying to protect him from taking on too much and nursing him back to health when the bad guys get the best of him.

I wouldn't take it as an offense, but a compliment if one is seen as one.

Absolutely, I love ESFJs as well. My mother was one, and I work with one I really like. It's just frustrating to be continually told you're something you're not. I can tell that you (and others) have genuine intentions, but I also have a lot of trolls who follow me around and try to upset me, so it's hard sometimes to tell who's interested in general conversation, and who just wants to pick a fight or waste my time, you know?