Your loss of bees was a failure to requeen after the swarm. Yellowjackets can’t/won’t rob a healthy, queenright colony. If the bees swarmed after June, the swarm will not have enough time to build up population to collect stores and build overwintering bees. Old beekeepers saying,“Swarm in May, worth a load of hay, swarm in June, worth a silver spoon, swarm in July, not worth a fly.” Save any stores for your nuc in the spring.
You don’t close entrances, you reduce them. Even after taking all the honey and leaving open screened bottoms in Iowa, 25% of colonies still survived. Cold doesn’t kill bees, winter moisture does!