

Because softeners have a maintenance cycle, most people want to just have one that serves the whole house rather than little ones for each appliance like some people do with water heaters. The pouch in the cistern is definitely not totally effective, but it’s a small upfront cost as opposed to the cost of plumbing in a system that costs hundreds.
As for rainwater, there are two parts there: the idea itself and the ceiling storage. I’ve been wanting to do rainwater for a long time. It’s not terribly complex, but requires a bit of money and effort to set up. It’s usually almost pure unless you live in an area with awful air quality, and if it’s only for the toilet(s) it should be fine anyway. On the other hand, the other part with the high mounted storage has to be done very carefully. Depending on your location, you would have to balance storage amount to last between rains, and as the storage gets bigger it gets extremely heavy. (A 50 gal drum of water is over 400 lbs, or ~190kg) A pump you have to service/replace every few years is way easier to deal with than trying to set up a structure to safely hold hundreds of kilos/pounds over your head in a place where leaks could mean heavy water damage to the house.




Pretty much. Brick is pretty good for load bearing so 50kg of water is not insane with the right hardware. Some anchors I’ve used were rated to do that with one screw, though I wouldn’t. I read your previous part as hanging from or inside of the ceiling but wall mounting should be good.
As for using heaters at all, it might not be purpose built, but it’s hard to beat free, even if they might have some quirks. The trick will be getting pressure moving in the right ways at the right time. The line coming from outside should give enough air to let water flow down easily to the cistern as long as you keep out contaminants/wildlife. A first flush and mesh should be able to manage that. And while I haven’t worked with ones like the top pair, ones like the bottom image, in grey, usually have a port at the bottom for draining/cleaning and you could cap off the cold water port, meaning you wouldn’t have to make extra holes. The hot water outlet port is usually at the top of the inner tank so it’d be fine.
The one thing I’m spotting is to remember the supply line has pressure and a very large supply, so you will have to make sure to either have a one way flow check valve or some other measure to keep it from sending water back up toward your rainwater inlet when using the piped supply or you could end up with a new pool where you don’t want it. Otherwise, very cool idea.