True, I've had a huge financial setback in terms of student loan repayment because of how many books I've scratched this past year. 50X, and Holiday Gifts actually weren't bad to me. 200MS, Lucky $200k/yr for Life, and $20 Flamingo were the culprits with some of the most disgustingly bad return rates I've ever seen. Even though Lucky $200k/yr for Life books cost $1,000, I swear I've encountered books of it that may have returned $250-$300/$1000. That's how bad they are.
It is certainly possible all jackpots have been destroyed, and none remain. Then again, with 3 jackpots posted it is also possible an earlier jackpot is out there. I'd personally bet the lower jackpot has been destroyed because of the likelihood of tickets being removed/destroyed during the transition of new lottery games, and 50X The Cash being booted out of machines and into independent retail stores/food stores.
As Zzzplayfaster recommended, it would be smart to only buy tickets in certain ranges. I think we concluded anything in the 610k+ range could hit, especially 620k. If a jackpot is to be found on 50X The Cash, I'd assume it will be the "last" jackpot printed in the higher range. It would be ironic if one of those low number books produced a "throwback" jackpot prize that was never found. $10 Holiday Gifts might just do this...lol.
The nice thing about AK Kwik is that they will let you choose your books, so you can intentionally select books in ranges you think have better odds of hitting.
It is a true gamble. We know odds of hitting that last jackpot may be better than playing any random game. I mean, this type of information turned one of our regulars into a millionaire. Maybe you'll be the next FL LP poster to hit a jackpot. 
Edit: As far as my guess of up to 20 locations, that might be a large estimate. That's probably including those "Far out" stores like Lawtey Community Food Store which i picked up two books of 50X The Cash. Apparently that store also has Power 8s. Stores like that in super rural areas tend to keep stocks. Across the state, it wouldn't surprise me if a few other stores like that had 50X. Then again, perhaps AK Kwik "pulled" their supply, and is now the primary holder of almost all 50X books. Those books they gathered probably got pulled from other lottery reps, and sent there due to the traffic/demand.