Yes, you can sit them in autumn without sitting them in the summer. The autumn series is not just for resit students, many first-timers are appearing too.
No, it is unlikely you'd be able to go to uni in the same year (if you mean a UK uni) since unis start in Sept/Oct, and the winter series results don't come out till January. Unless you already have your A-Level or equivalent qualifications with you by August of the year in which you want to begin your uni, you will probably have to wait until next year.