Blaming the Pioneer Plaque for the progressive degredation sounds like it makes sense at first, but the point of the Pioneer Plaque thing is that this Voldemort is supposed to be smarter than canon Voldemort, and a Pioneer Plaque horcrux superior. That theory makes the Pioneer Plaque horcrux inferior.

Smart people still overlook things. A lightspeed delay problem in horcrux syncing would not have come up ever before, so it could have been easily overlooked even by a very smart person, especially one that is not scientifically oriented. If he had been more scientifically oriented and been otherwise interested in Muggle space programs, this possibility might have occured to him and he could have tested it with the moon missions, if he had come up with a way to detect anticipated problems.