GM,
Weekday 2-a-day's will work nicely if they meld with your schedule better. The weekend long session is the key ride to keep unbroken.
Yes, it is okay to substitute road rides when necessary instead of mtb rides. Again, the longer rides are the key ones for being off-road. Limit substituting the longer rides. Shorter rides all work fine on or off-road. The intensity target is the most important aspect of shorter rides.
You are on the right track to making this plan work optimally with your schedule. Well done.
Lynda.