Is it permissible for me to shorten and combine prayers upon my return to Yanbu for three more days after having traveled from it to Makkah or Jeddah and spent two days there, given that I had intended to reside in Yanbu for fifteen days and had already shortened and combined prayers during the first three days of my stay?

If you travel and intend to stay in Yanbu or any other city for four days or more (not including the days of arrival and departure), then you are a resident and must perform the prayers in full. If you leave the city where you resided for another city and intend to stay there for four days or more, then you are not permitted to shorten or combine prayers there. You are permitted to shorten and combine prayers on the road between the two cities if the distance is a distance that permits shortening. Upon returning to the city from which you traveled, you are permitted to combine and shorten prayers unless you intend to reside there for four days or more.