R’ Zalman–I know girls, myself included in my dating years, that it was uncomfortable to be watched while getting into a car–no issues of not dressing tzniusdik–just a sensitivity, and I appreciated when the bochur wouldn’t stand there and wait while i got in. Not every tzniusdik girl/woman would have this issue. But is wasn’t the issue of lack of tznius just a sensitivity.
As far as hotel door-she should not go in first. He can go in first, and still hold open the door behind himself (don’t need any special degrees or sugyas to figure it out) so she can come in and he should continue and all laws of shulchan oruch are kept, no chukas hagoyim, no chivalry, just plain and simple mentchlechkeit.
Yes, we as yidden follow the shulchan oruch. But for example, just because the shulchan oruch doesn’t point out “rochel bitcha ha’ketana” how wide your smile should be when greeting guests into your home, we need to also have seichel when it comes to mentchlechkeit. If a bochur is going to delve into each potential mannerism that might be chivalry when with normal seichel and of course a well versed knowledge of shulchan oruch to back him up, he might be left in the hotel by himself.
That was my point and I think many others.