I've watched the series and read the script books numerous times and I never heard the explanation of the halo - which I find very interesting, thanks for mentioning it! I was not aware till now that the halo Mrs Bartlett saw was anything more than a figment of her imagination. I am pretty sure that the explanation was not given anywhere in the actual show.
You often see Isobel dropping in to visit Violet and I never got the impression that she had been driven there. In general, I think that a century or two ago (and earlier too, of course) it was normal for people to do a lot more walking to get places than they do today. Jane Austen's characters think nothing of walking a couple of miles into town to do some shopping or over to the next estate to visit the neighbors.
Violet, being much older, probably got driven places more than Isobel.