Thanks for the great questions. I have not read the Norwegian Oly Committee recommendations but suspect they are aimed more towards conventional SKiMo race distances of a few hours and may not take into account ultra distance events like you are training for. As with any long distance event; duration or volume of training is more important than intensity and the skier with the bigger base volume will have an advantage in both training and also racing.
Your idea of building volume from 2×6 hours/week in the early base to 2 even longer workouts is a sound one. However, I think that only 1 mega big day of 14 hours would be wise and sufficient in a normal week’s training near the end of the final pre-competition build up period. It is not necessary to recreate the volume of these ultra distance events in training. The recovery from such grueling workouts may take too long and may cause to loose fitness between good training bouts. Of course this is all very personal and needs to be considered in terms of your training history.
I have successfully used back to back big days in training ultra runners. For you, this might mean a ski day of 10-12 hours followed by a good sleep and meals and then doing one of 6-10 hours the next day when you are already depleted from day 1. Training in this glycogen depleted state is a very powerful training stimulus for aerobic adaptation. Be sure to allow for some good recovery after over reaching like this.
Another option would be one that top XC skiers use often in training camps. Set aside a week for your own training camp. be sure this is at least 5-6 weeks out from your race. During this week you will train like you can’t in a normal week. Overreaching every day with a mix of intensity and volume. Then allow a 10 day recovery period after this overreaching block to allow the training to have its effect. The recovery period should cut volume first by 70% then gradually bring the the daily volume up to around 50% of the camp volume as you feel better. In your camp you will train 2x/day. You will always be in a depleted state so never feeling great. In between workouts you will do nothing but eat and sleep. The total training load will be 30-50% larger than your average for the past few months. This is a very hard week.
The role of high intensity training diminishes as as the duration of the event increases. This does not mean it plays no role but the benefits from high intensity training for someone doing a 30 hour event will be nothing like the benefits for someone doing a 30 minute event. In your event you will not, or should not, be using such high outputs. Skiing hard in your 30 hour event will have a big negative effect on your overall result because your event is all about energy management. Yes, you might gain a few seconds as you blast up one hill but it is very likely that you will then loose MANY minutes later on because you foolishly wasted some precious glycogen stores.
If you plan on using some Zone 4 interval training, which does have benefits for anyone who is ready for it, I suggest starting that early in the base period when the volume is low and you can adapt to the particular kinds of stress these workouts impose. Waiting until you have a really bog volume load before piling on top of it a hard interval workout is likely to cause you problems with handling the load.
My recommendation would be for you to use long uphill Zone 3 (instead of shorter Z4) training instead and do this also all through the base period. Start with 1-2×20 min and build 1x 6o min at the top of Z3. This will really push up your endurance by raising the lactate (anaerobic) threshold closer to your max.
I hope this helps