Wow. Don't envy your situation ShellThom. I'm thinking that you should possibly be calling in the Dog Whisperer Cesar Millan as opposed to a wedding planner right now as you've got a textbook redzone case here that needs to know some rules, boundaries and limitations.
And there are a couple of red flags here as to how previous events have panned out. And every wedding planned is complicated just doing the ceremony and reception let alone such a huge planning event as this.
My son attended his mates wedding in South America. The wedding itself consisted of multiple ceremonies. A group of friends rented a house and had a wonderful time. But the difference here is that the guests organised it themselves leaving the bridal couple to deal with all the ceremonies.
I think this is the key thing - by all means invite people to your destination wedding but let them planning plan their own trip. It would be hard enough trying to organise and wedding and a holiday/honeymoon for yourselves never mind for 20 or so other people. It would just be way too much on your plate IMHO.
Vegas and Hawaii seem popular bucket list destinations for al lot of people so why not get married in Vegas & honeymoon in Hawaii which would give people more options eg some may just do Hawaii only, others Vegas and Hawaii and some may do Vegas then do their own thing before or after.
I would set the dates & try to do it outside US or Australian school holidays as fares will be the most expensive then work out when you'll be in Vegas & when you'll be in Hawaii. Maybe do a Facebook page or some other webpage you can update regularly with details for the event complete with the dates & where you'll be staying.
* Leave any travel arrangements up to the individual to book - don't have them pay you etc as that's just too much work
* I wouldn't do an Amazing Race type mystery scenario as most people want to know what they're doing, when & how much
* Special dinners, sightseeing or touring can be done closer to the time once you know who is coming
* A lot of hotels in Vegas would have wedding packages where all the important stuff is taken care of by a wedding planner so I'd leave that bit to the experts
Most importantly make it clear to your future M-I-L that emotional blackmail will not be tolerated under any circumstances. As they say in the South "them days is over"!
If she threatens "to cry if (insert excuse here)" tell her "what makes you think you're even invited"?
If she does come instead of Elvis walking you down the aisle it could be The Terminator so if she plays up it'll be 'hasta la vista baby'.