If you’ve not watched this yet, please do. It might answer most of your questions: https://uphillathlete.com/heart-rate-drift-test/
A negative Pa:Hr is what you’d expect in an out and back test. That’s why we recommend against that kind of a course. Try it on a loop course even it is rolls some. Keep less than 50m vert/km. Rather than adjust starting Hr yet try a new course that is not uphill one way and down hill coming back.
The pace is going to be what the pace is. If it is 9min/km that means that this is all the ATP your aerobic system is capable of producing right now. That’s what is called your aerobic capacity. Go faster and the balance of ATP production will have to come from the anaerobic system. People with out a solid history of aerobic base training will usually see this result. The good news, and you can read it all over this site (testimonials and forum posts), is that this is easy to fix. it only takes patience.
Good luck and report back.