Thanks for writing into the forum. Great questions. It sounds to me like you did a great job of training for this skimo season.
You say: “in races, my energy and endurance is very good, no bonking, no lactic acid. I am not sore after races” What’s not to like about that.
I suspect that your HR is higher in skimo higher than you are used to for 2 reasons. The races are shorter and you change modalities between uphill and down hill. Secondly this is a quadrupedal sport so there is a much larger muscle mass involved in propelling you up the hill. This requires more oxygen. More oxygen can be delivered by an increase in the stroke volume of the heart but that’s not likely to change more than 3-5% with the higher intensity training you’ve been doing. So the only option to deliver more O2 to the muscles is to increase heart rate.
The fact that you can maintain almost 90% of your max HR for 2 hours is an indication of very high fitness. Hence my comment above about your doing a great job of preparing.
This is clearly demonstrated by the treadmill run you just did. Congratulate yourself!!
However, and this is really important if you want to keep feeling this way during races. DO NOT GET GREEDY WITH YOUR TRAINING, ESPECIALLY IN THE RACING SEASON. You see these gains and you think there must be more to come. By everything you have said your body is now at a fitness level that it has never been at or at least not for many years. To expect expect it to be able to keep increasing indefinitely. This is a classic mistake. Most people think to themselves: “if all those intervals were good for me before I just just do more of them”. This runs a big risk of eroding the aerobic base of support that is so key to the results you’ve been seeing. If you have more than 2 weeks before the next race AND you plan to do several more races this season I would recommend adding more volume in this mini training block with maybe 1 interval session. I give you this caution because I have seen the wheels come off the fitness machine alarmingly fast and the athlete go from the state you are currently in to a mere shadow of themselves a week later.
I hope this helps.