I would suggest there is no reason not to get an inspector to look at the hive and he might come up with another thought that might be behind what happened, or he might also agree with some of us.
The colony didn’t swarm as when that happens only about 1/2 the bees ‘bugged out’ so it sounds like they absconded, when the entire colony leaves the hive.
As there was a few dead and some stragglers which I assume were alive it doesn’t sound like poisoning as the cause. Most poisonings you will find the brood box really deep with dead bees as well as on the entrance and the ground under the hive and you haven’t mentioned that seen so I would be inclined to cross that off the list as possibles. Any inspector worth his keep will look at that as a possible.
My thinking is that something trigger the colony into swarming and it could have been the flooding and they decided home wasn’t all that safe a place to live. Flooding an happen in genuine flow hives as well as the ‘counterfeits’, and it is not always the bee keeper at fault. Sure, if you rush the extraction it will go ‘pear shaped’ but even if you follow all the cautions flooding can still happen. I have one of the four Flow Hives I bought that is more prone to flooding than the others and I have done everything suggested to fix the problem.
Cheers Carola
At least with a genuine product you have some semblance of after sales support and a warranty if any issues are due to manufacturing faults.
Won’t get that from a counterfeit flow as I doubt the copycats can manufacture with the required precision, and would you really want to eat honey coming out of plastic of unknown origin?
@Carola, @Stevo makes a very interesting point. I had a closer look at your Flickr video, and the frames don’t seem to be stamped with “Made in Australia” or the frame model number. Genuine Flow frames always are. Where did you get the frames from? Can you see a “Made in Australia” stamp on them - perhaps the light wasn’t right in the video.
I am really sorry that this has happened to you, and I am sorry for the loss of your bees. These things are hard. But you came here and asked good questions. You obviously care about your bees. To me, that means that you will be a great beekeeper. I really hope that you try again. 
My husband ordered them from Amazon. I don’t see any stamps on them. I found the plans for the hive on Etsy and he made the boxes. Sorry, if people are put off by that. Maybe, it’s not a good idea to seek information on the Flow Forum.
Thanks for the articles 2Dawm_SD. I wish I had read those earlier and not been as cavalier about harvesting so much honey at once and also planned to do the inspection before and not after harvesting. That was really stupid. I had planned to treat for mites as the weather has almost cooled down enough but perhaps that would have also been too little too late. I suspect now, the hive wasn’t as strong as I thought it was due to mites and maybe some robbing. Frames were pretty empty, the remaining honey was also gone. One frame with newer comb looked like a couple of small chunks were gone. But I don’t know if wasps or something else did that after they left.
Super bummed if this could have been avoided with more education and experience.
I really appreciate the helpful information though.
Will think about trying again next spring. I’m wondering if Sascatraz bees might be a little more fastidious about their home as well than another breed, also contributing to them wanting to leave. Maybe I’ll consider Dreamweaver or another breed next year. The kids initially wanted the gentlest breed but now are all for Russians as they want them to be able to fight off mites be strong.
Sadly, it sounds like you may have been conned with forgeries. Once again, I am very sorry that this has happened to you. If you have the information about the seller, I am sure that @Freebee2 will pass it on to the Flow team. Flow does sell genuine Flow hive components on Amazon, but so do some criminals. This won’t help you, but it might help prevent the same thing from happening to others. What they are doing is illegal - you didn’t do anything wrong, you are just a victim of their cheating.
I just found a picture of genuine Flow frames on another thread, so that you know what I mean about the stamps on the end of the frames:
You are being too hard on yourself. 
We are all human, and we all make mistakes. Not a single one of us is perfect. The good thing is that you came here, and you learned from it. If your plastic frames are fakes, then all of the reading in the world may not have prevented your hive from flooding. The fake frames have design flaws in them, which make them hazardous for bees. You were not stupid, you have just been badly harmed by criminals.
Please don’t feel that way! We all love bees here, and most of us are interested in any kind of hive, not just a Flow hive. I admire people who build their own equipment. I just don’t have the skills and tools for that. You are very welcome here, especially as you are so willing to learn and ask questions.
Saskatraz or Russian bees would be excellent in your climate. I bought packages from Mann Lake this year - they have excellent bees in the Spring.
I really hope you try again next year, either with traditional wax frames, or perhaps with some genuine Flow frames. I wish you and your family all the best!
Just an update–
The bee inspector came today and he determined the demise of the colony was clearly PMS. He was able to show me some indicators of the demise with the brood and he said there were no signs of any other pests or decease.
Still feeling very bad about not treating them earlier. Our temps were in the 100s and I had some back issues that contributed to my failure to check on them and treat them in time. I also overestimate the Saskatraz bees abilities to keep the mites in check. What is the best option for treating mites in high temps for future reference?
Well done on getting an opinion from a local expert. 
If you want to be organic, oxalic acid vapor (OAV) is probably the best option. Otherwise Apivar strips are very effective if rotated with other treatments. However, you can’t treat with a super on the hive. I am going to link a very good, but slightly outdated (about OAV in particular) article, which gives you more information than you would ever want to know about Varroa and how to treat your bees:
Hygienic bees can keep down the numbers of Varroa a bit, but in my experience, they have not controlled the numbers well enough to rely purely on their genes.