Let's say that low-SHBG guy is identical to you in all regards except SHBG. You both inject inject 100 mg TC every Monday, and after steady state you measure serum levels on Friday. You, with SHBG 70 nMol/L, measure total T as 950-1000 ng/dL. In this case low-SHBG twin, with SHBG of 7, measures about 800 ng/dL. Basically, his total testosterone is always going to be around 20% lower than yours. You do not and cannot metabolize/excrete at a lower rate. We've agreed that you're each absorbing the same amount of testosterone over any time interval. Therefore, at steady state, over any reasonable time interval, you must both be using/excreting the same amount. Furthermore, applying the law of mass action, Absorption_rate = MCR * free_T, means you both have the same free testosterone over any reasonable interval. This is how you work backwards to determine low-SHBG twin's total testosterone—we know he must have about the same free T as you.
... There's zero proof anywhere to date, that can demonstrate how bound testosterone acts upon the bodies cells...
As I mentioned above, there are some known uses for SHBG-bound testosterone, and they may contribute to suboptimal results by low-SHBG guys. However, the evidence so far seems to be saying that this activity plays a relatively small role in the consumption and elimination of testosterone.
...but one thing is for certain and that is someone with high shbg will keep released testosterone in circulation longer than someone with low shbg.
No, free testosterone is what drives this. If you and your low-SHBG twin have the same free testosterone levels then you will both be metabolizing and excreting testosterone at close to the same rate.