Welcome to the forum, glad you joined.
I strongly endorse you building your own open air walk-in coop, preferably in that shaded area. Heat can kill, your cold isn't bad at all for chickens. I can appreciate the time involved but keep it as simple a you can and keep yourself happy. The chickens don't care how pretty or complicated it is. You might follow the link in my signature to get some of my thought on space. If you are buying new materials standard dimensions are 4' and 8'. Using those can reduce cutting, waste, and erection time.
Losing three out of four is pretty unusual. Not sure what happened. Stuff happens, but if you lose any of the new ones give us some details and maybe we can help troubleshoot your set-up. Some people at the feed store know what they are talking about, some don't.
I personally like a dirt floor in a walk-in coop but that's just personal preference. A critical factor is that the coop needs to stay dry. A wet coop or run is an unhealthy coop or run. If it stays wet much over two days it can start to stink, much longer and it might harbor diseases. So definitely elevate it. Slightly damp in't an issue, it's when it stays wet you have issues. I doubt your flood irrigation will cause an issue in the yard, you probably dry out pretty fast.
One of the potential issues with a wooden floor is that if the bedding stays damp the wood can rot. Some type of protection on top is probably a good idea, maybe linoleum or paint. If poop builds up much it can stay damp.
If you leave an opening under the coop mice, rats, snakes, or other things might set up housekeeping. If the chickens can get under there they will pretty much stop that. But if a chicken can get get under there you need to be able to retrieve and egg or a chicken that doesn't want to be caught. So if you raise it, raise it enough that you have access.
Your block wall may or may not cause issues. Chickens love to perch, it is a favorite past time. It is possible they will fly up there to perch. Who knows which side they may hop down on. If this becomes an issue for you (it may not) you can put wire mesh about 6" high around the top of the wall so they don't have a good place to land. Just something to keep in your back pocket.
I would certainly keep that tiny coop. It can come in handy if you need to isolate a chicken for some reason or maybe for future integration.
Good luck, I think you will do fine. And once again,
