To me it sounds like the op is trying to make trouble and the mods are having a slow day! What exactly is the point of this thread and why has it not yet been taken out?
What is the difference shabbos and YK? that is like saying do people who keep a kosher home but eat dairy out, also eat chicken out?? the answer is yes probably.
Depends if they believe it is wrong and simply do it because they can’t help it, or if they believe it is okay. As Wolf said, it is likely that some do and some don’t.
And probably not. People don’t think it’s okay to text on Shabbos; they just don’t respect Shabbos. I think even teenagers still respect Yom Kippur. Those who are “frum” but only text on Shabbos probably wouldn’t on Yom Kippur, not that that made them any less of a Mechalel Shabbos.