Many of your questions are covered in the rules on the linked page to the IBR and would encourage you to read through them because the rules are very specific and proscriptive. A I add on some information/opinion and/or may not be in the rules. Also, it can be said that some common principles learned when doing most any rally--not just the IBR.
1. What is the advantage of having more than 1 GPS? Do you leave GPS#1 always navigating on the next checkpoint to make sure you stay on overall schedule, and then perhaps GPS#2 is sending you to the next bonus?
One GPS has the main route or to the CP and is left fairly steady. It's the one you really watch to make sure you are going to the CP in time. The other GPS, besides being a backup if your first one fails, can be used to play "what-if" scenarios. What if I go to x, z, then y. What if I add in a & b? What if I skip x, take this interesting side road, and hit Topeka at 2 a.m. instead of 7 p.m.? If you decide something on the what-if GPS is worth it--you then change the main GPS. At least that's how I worked two GPS.
I also had two different model GPS set a bit differently. Sometimes the difference would be interesting and I'd make some route tweaks to my advantage as a result (e.g. that gravel road that my 470 like is actually quicker than the 590LM paved road)
2. The definition of Rest Bonus is not entirely clear to me. Is there a minimum amount of time per 24 hour period (or other) in which the motorcycle may not move? If so, what amount is that?
You get to a place and get a receipt. You have to get a receipt at the
same place (or stamped with the same city) x hours later (typically 4-8 hour rest bonuses on sliding scales). You can move your motorcycle during the rest bonus, but why would you drive around town instead of going to a hotel and get sleep? Rest bonuses really are used (and cherished by riders) to get to sleep as quickly as you can. If I had a 8 hour rest bonus I'd probably get 6.5 hours sleep after getting checked in, quick shower, eat something, go to sleep. On the other end I'd look over my route on the laptop and get back on the bike shortly after the 8 hours for a receipt.
3. Are competitors required to submit photographs that prove bonuses or strings to the rallymaster as they take them, or at the checkpoint, or other? Are there safeguards in place to ensure nobody gets stupid with Photoshop?
Yes, with a provided rally flag according to directions of the bonus. There are safeguards in place to reduce Photoshop shenanigans. I've never heard it EVER being an actual thing before.
4. If a competitor follows the rule whereby he renders aid to another rider or a member of the public in an emergency, and that causes him/her to miss the checkpoint time deadline, is he/her still disqualified?
Covered in detail in the rules.
5. Are rally teams competing for the same prize as individual riders? Do teams have to follow more strict rules?
Rules are more strict for teams and two-up riders and covered in detail in the rules.
6. Is the selection of the bonuses engineered such that it is impossible to grab them all? IOW - are there so many bonus point opportunities, that each rider has to develop/design their own ride/routes/stops/etc?
Yes, it's impossible...by many factors. My experience has been 10-25% are gettable. If this rally has the same list for all three legs...maybe they'll get a higher percentage, but still never get them all. Only one rally in my life did I nearly sweep the table of available bonuses. A Utah 1088 I rode "the alternate route" and got all but one from my list...and surprised the rally master. It was like eating the whole buffet, but leaving one shrimp, burping, and saying, "I'm full now."
7. How to the riders determine where the bonus places are? For this years event, in which they are looking for roadside animal art, do they just pull out their phone, google it up, and hope for the best?
The receive specific GPS coordinates on a thumb drive that are VERY VERY close to the thing you need to photograph. The IBR coordinates have always been impeccable. They double and triple check them as well as QC their rally packs.
8. I can't see where there is a minimum amount of miles that need to be ridden, other than the shortest route from checkpoint to checkpoint. Why then is the odometer certification and other odometer related rules necessary? What am I missing? Is it to prove that the competitors actually rode the motorcycle to the various places (as opposed to throwing it on a trailer)?
No minimum miles. In about 2007 or 2009 they instituted a minimum point to be a finisher. Odometer readings are important for validation of riding routes claimed sometimes, but also an interesting data point to see which people rode harder vs. smarter.
