Definitely yes.

In fact Ms is a much better translation of 小姐 than Miss. Even after marriage Chinese women will use 小姐, so like Ms it applies to married/unmarried females, just as MR does with males.

If 李小姐 married 王先生 she would still be known by everyone as 李小姐.

王太太 would be used only by strangers.

Yes, it is now.