Not sure how you came up with DC 31.
Changing your example for a minute, say the scroll is a 9th level Teleport spell. (to allow you and 3 other members to be affected) The druid only has a 12 Int.
Using UMD to bypass rule A&B, the druid would need to roll a 29 or better (base 20 + 9th level caster).
Your effective ability score is 14 (your UMD check of 29 - 15). Since the spell needs an Int score of 15, the druid has to emulate the ability score as well. What would the DC be?