You keep missing a part that clearly states "in person".
A person working under the supervision of a holder of a mechanic or repairman certificate may perform the maintenance, preventive maintenance, and alterations that his supervisor is authorized to perform, if the supervisor personally observes the work being done to the extent necessary to ensure that it is being done properly and if the supervisor is readily available, in person, for consultation.
I think you are going too deep into this interpretation. What this is saying is that the mechanic can not legally call in a repair and sign off on a task without inspecting the work. And the mechanic can not legally (or ethically) allow an owner to perform tasks at which they were not trained or qualified without first ensuring that they are available in person to ensure the task was complete correctly. Once training has been done, or they have ensured the owner/worker can competently perform the task without direct supervision than the A&P can delegate the task for completion up to...but not including...the inspection for the item to be returned to service.
Add another layer to this...a large Repair Station is completely filled with Non-A&P technicians performing almost ALL of the work. The A&P is available (often in his cube or in another area i.e. inspecting a task), but definitely not over the shoulders of the workers. What happens if he goes to lunch? Does the whole shift stop? No. GRANTED they are under the authority of the repair station license so they are given more flexibility than an owner/A&P situation.
Now, lets add some beans to this casserole...Say I'm removing my canopy top (Grumman Tiger) to replace the window side panels and replace the teflon track runners. The replacing side windows is fully authorized by a private pilot under CFAR 43 Appdx A Preventative Maintenance, and the Teflon runners with the right tool is a very simple job. However both are structual and require an A&P certified technician to sign off on the task. Now if I had a trusty mechanic that I knew and he knew me, would I need him look over my shoulders or be in the hangar while I perform this task?
No. Why? Because I can take a item out of service and make it un-airworthy without a mechanics license, there is a whole thread dedicated to this.
Now, by making the repair, can I place the item back into service? Absolutely not. Only the aircraft mechanic can, and he can delegate as much as he wants knowing full well that if something goes wrong its his certificate.
Now, can I as an owner put the aircraft all back together without having the mechanic look at the quality of work? No, because how else can the mechanic ensure that the job was done right?
So how do I find a happy balance? If the A&P technician does not feel comfortable with the owner performing the task, or if the owner was not provided training on how to complete the task properly, then the technician has a responsibility (legal and ethical) to avail himself in person to provide services.