I'll take the second question (since it's easiest
).....
I doubt you'll be asked that, but IF you are: you tell them yes you'll stay. (even if you know you won't)
You have to look out for you. But of course you can also always tell the truth, even though that truth puts YOU at a great disadvantage.
If you tell them "no, I'd still be looking for full-time" -- then they likely wouldn't hire you in the first place. What do you want to do?.... put a sign on your chest saying "DON'T hire me. I won't stay?" and talk yourself OUT of a job?
As for the first question, you said yourself the duties of your job had changed. But of course, I'm sure you know that doesn't really explain why you resigned. People who work have their duties changed all the time. More responsibilities added, duties bait-and-switched on them....and they don't quit. They stay until they get a new full-time gig.
Hiring folks and interviewers get why people might be working part-time, while looking for full-time.
They have a harder time with applicants who quit their last job with no job lined up. It looks like you're not loyal, won't suck up certain conditions, and will quit on them also.
Of course there ARE perfectly legit reasons why people quit jobs with no new job in hand....but it can "look bad" ...which is why answering your first question is tricker than the second question. And why I left the first question to others
....