Your original design is the most cost effective, efficiant, and I believe safe way to solar dwh. I work with heat exchange, it's our lifeblood.
If you develop a leak in your HX, barring some un-natural force, only 2 things will happen.
1) You will have no hotwater supply/pressure.
2) Your buffer tank will flood with cold water.
Further, I fail to see a true risk to human health. Who is drinking HOT water?
It is always recommended NOT to do that.
Double wall HX do not work well. You need whatever your largest gallon demand is soaking in that HX/tank to really take advantage of your low cost system. A double wall HX that size is costly.
I to want system like this. I want to run at 70f and soak the condenser coil on a modified inverter ASHP. I will need a BIG tank.
Best of luck to ya, I have some good friends in Trial.
