There’s a lot to unpack here so let me begin by saying that we can’t tell you that you are overtrained or not. Just having a lower heart rate in the last test is not conclusive of OTS. Do you have some of the other symptoms of OTS?
Next: I think there is a much bigger chance that your tests were no done well enough. I’ve never seen an improvement of 20 beats in 2.5 months. That would be remarkably rapid and I guess it might be possible, I’m suspicious.
Then: As with any experiment the conditions need to be as similar as possible to the previous test. If you were a bit tired that could easily result in the lower HR although I still think the second test is fishy.
Also: The Big Vert plan is for advanced runner with a good aerobic base (as explained in the plan’s description). We recommend it only for those already comfortable doing around 50+ miles/week. This does not meant you once ran 50 miles in a week or that your peak weeks might hit 50 miles. I means consistently running 50 miles a week without become excessively fatigued. You are right to question your use of this plan if you can’t keep your HR in check.
Lastly: This initial section of the Mt Si trail averages 35%. This is a STEEP trail in anyones book. I know very few, if any people who could run that gradient for that long and stay in Z2 unless they were taking tiny steps. Did you do a long warm up before you het that hill. The aerobic system takes a long time to come on line and if you head steeply up hill right out of the parking lot like trail does your aerobic system will not have time to come up to full capacity unless you start very, very slowly. 22min/mile is not slow for a 35% grade.
I hope this info an help you
Scott