Welcome to the forum. Thanks for your confidence in our services. I'll go directly to your questions.
1) The RNA and DNA direct tests were not designed for the purpose of early HIV diagnosis. They are commonly used that way, but there has been little research on test performance in this setting. The result at 10+ days after exposure generally believed to be at least 90-95% reliable. Most likely it's even better, approaching 100% -- but without more research data, it is hard to be certain.
2) This test result doesn't mention p24 antigen, so it is not a 4th generation test -- it appears to be a standalone antibody test (i.e., 3rd generation). At 18 days, it probably would detect about 70-80% of new HIV infections.
If you have another antibody test at 4 weeks, the combination of that result plus negative RNA at 18 days probably will be virtually 100% reliable. But for definitive results at that time, a duo (4th gen) test would be preferable; or have another antibody test at 6-8 weeks.
Here are a couple of threads that discuss test reliability at various times after exposure:
Best wishes and stay safe-- HHH, MD