Yes, the verification teams use Google Maps and other mapping software to verify rides. That is generally an accepted way to develop a route and have an idea if and when you need to document a 'corner' beyond the DBR every 300-350 miles or less. Your extra mileage is excellent and in the suggested range that the IBA likes to see.
Some observations; You say you are patterning your events after the 1088 and Cal24, but both of those events were endurance rallies, not Saddle Sore events. They were a competitive rally with points accrued by riders choosing a route based upon the bonus options, or collecting bonuses while riding the Main Route provided by the rally. Finishers were ranked by the total of their points, not with any concern over when they arrived at the finish, as long as it was within the 24 hour time window for the rally.
From your Aug 14th event listing:
"The 27th /28th Blue Butt Rally is about map reading and correctly answering questions along the
340 – 410 miles route is the name of the game. It is not a competitive event nor is it a race; one
fantastic ride. There is no scoring, ranking or even a time limit to finish. (Although most riders
typically finish in 8 to 12 hours.)"
I understand this is not for the Saddle Sore version. But if it's not a competitive event, who's checking to see if the questions are correctly answered and why? Especially if there is no scoring, ranking or time limit to finish. Typically this would be done for raffle prizes or extra tickets for such.
I've done rides like yours in the past, and dozens of competitive endurance rallies, including several Utah 1088s. You're just hosting a fun ride and that's great, as well as a solid charity to fund. If you are going to provide a specific route for riders attempting to complete an IBA Saddle Sore, then the riders need to know that the final responsibility is on them for paperwork, receipt collecting and turning it all in to the IBA for a certificate. As well as paying the IBA fees. You could certainly help by having someone not riding the event be on hand to witness odometers at the start and finish, offering a convenience for the riders. Easier at the start. Some riders may need to find their own finish odo witness unless staff will be waiting for all riders to come in. Which sucks when that guy that never calls when he drops out is involved.