#1 Legionellae can not grow in the collector or the tank because the longest amount of time the collector will go without reaching temps above 150f is about a week.
Legionellae can still develop, it's a bacteria and given the right conditions could rapidly develop. The Regs here will not allow any contamination or possible contamination to backflow into the cold water supply. so that could depend on the materials the water is sitting in ie the solar panel piping material, the storage tank
#2 What is it that your governing body is worried about that is going to back flow from the collector? In an active-direct system it's just potable water in the collector loop. We already discussed above that bacteria cannot grow in the collector loop because of the high temps. How is it any different than having a two story house with a hot water circulation loop?
The Regs here will not allow any contamination or possible contamination to backflow into the cold water supply. so that could depend on the materials the water is sitting in ie the solar panel piping material, the storage tank. A hotwater circulation loop (called a secondary return here) would form what is called the distribution circuit and the regs state the water should be stored at not less than 60c and distributed at not less than 55c. The recommendation on scalding grounds is that it should be 50c but that thermostatic mixing valves be used at every outlet to prevent harm to persons particularly the young/old, vulnerable, infirm and sensory challenged. So that shouldn't pose a problem here.
#4 No, they aren't similar in price. The direct system is far cheaper (all things considered). You quoted $5987 just for the materials. I quoted $4000 for materials AND installation. Your system is twice the cost. How is that similar?
Ahh but I quoted the price an installer or shrewd diy-er could buy it for also, and the installation cost would depend on local circumstances. Most hoses here are at least 2 story which is more risky to install on so it would cost more here. So the cost of $3592 +$1000 for installing it isn't far off your price of $4000 and this is for a premium branded product (not that i'm saying yours isn't). For a non branded system it would be up to $1000-$1200 less. I'm often under cut by Â£1000 - Â£1500 but their the people that phone up a year later when the company that installed it have done a runner or folded and reset up under a similar name without the liablities, with a system that was shodily installed and on the cheap, non solar rated components, such as central heating pump, pressure vessel soft soldered pipe work, heating pipe insulation, pressure relief into the house,... I'm sure you see your own horrors there.
Plus I also pointed out that you could install a simple indirect system for $1500
I can accept the no need to have an internal tank arrangement and have a system like this for example..
I'll not link direct to them but you can work it out as I am not advocating their product just citing it as an example.
The cost of something like that is far cheaper more in the region of $1500