Since you mentioned common North American wire gauges, I will give you the facts based on North American code.
From your responses so far, you seem resistant to hearing the facts of Code, and seem to want to argue whether Code makes sense or not. I'm not going to argue something that isn't my decision, so I'm not going to go into the details of "why".
Suffice it to say the people on the Code council are just as interested in saving money as you are, and do not waste your time or money with unnecessary regs. It's there for a reason, even if you don't know it. If the reg seems stupid, an honest search for facts will reveal it is not.
You can't parallel
No, two 8 AWG is not better than a 6 AWG.
Paralleling is allowed in certain heavy industrial applications where wiring is subject to professional management, approval chains and regular inspections. In those cases, you can't mix sizes. And each conductor has its own overcurrent protection. Paralleling requires very special panels made for paralleling. Doing this in a regular service panel creates perfect-storm conditions for a stupid person to blow up the panel.
Hot on a ground wire is out of the question
Putting hot on a ground wire is a no-no. It doesn't have any insulation! You shouldn't be horking together your own insulation. Ordinary wear and tear in the panel can expose that insulation and then kaboom.
Yes, there's an outer cable sheath - but the sheath is there for physical protection, and is not designed for insulation.
Ground needs to come with conductors
On a new installation, grabbing random ground off a different circuit is a no-no. You need to install an appropriate ground wire with your appropriate conductors.
However, there is an exception for retrofitting grounds, i.e. to an installation that had been (legally) done in the past when grounds were not required. In that case, the ground borrowing you did would have been permitted, provided of course that a 10 AWG ground is acceptable on a 70A circuit. I doubt it is.
6 AWG seems a little light for 66.6A
I know the manufacturer says it can use 6 AWG. That doesn't sound right, and since their incentive is to get you to buy it, I don't trust them.
It boils down to the temperature of your terminations. Most wires are rated off the 60C column, where 6 AWG is good for 55A, because of rules in the Electrical Code. If Code allows it and all your terminations are rated for 75C, then you can run 65A on 6 AWG, and a 70A breaker is allowed as the "next size up that's available". A better choice would be 3 AWG AL.
Maybe your heater already splits the heating elements
Many hot water heaters subdivide their heating elements into smaller sections, e.g. two of 33.3A. And then they allow you to supply power to each heating element section separately, e.g. two 40A circuits with 8 AWG. You might check if that's possible.