For Sunday you will just have to get as much sweet spot in as possible. To get 185 TSS in 2.5 hours is going to take a lot of work, especially if you had a hard Saturday sweet spot group ride. You are looking to get in 75 TSS per hour on that ride. Try to get in the ballpark. The rides are not super long but there will be plenty of work in the 2.5 hours.
These workouts can be flipped flopped as well. So you might want to try to do the higher TSS day on Saturday when you fresher. Then Sunday can be a little less.
Absolutely! You can use your heart rate while mountain biking. Actually a better way to measure your effort while riding off road as the heart rate will be a bit more consistent with the effort while the power will be all over the place.
Yes you can do HR instead of power on zone 2 days as well on zone 2 days. In sweet spot 4 we actually have zone 2 rides labeled by heart rate. This is to make sure you do not go too hard and keep the heart rate down.