Sorry you got the runaround! Of course it is safe to give your dog the Trifexis, even though it is 10 days late. Some people give their heartworm preventatives every 45 days. This is no longer recommended because the heartworm cure manufacturer is having difficulty keeping up with demand so it is safer to stick with the 30 day protocol now. But if you do your research on the lifecycle of the heartworm you will see for yourself that being 10 days late one time is not going to lead to a crisis.
Taking to vet immediately would be a really good idea