Sorry to hear about the hot water issue.
While I am not nearly as experienced as most of the guys on this forum, I would have a hard time believing that you problem is due to hard water buildup in the copper pipes. If you drained the pipes and water heater recently, then you may have some air in the system somewhere. Having personally drained my system many times over the last few months, I have to repressurize the system in a specific order or I will get low pressure in some fixtures. Try to turn the water off and open all of your fixtures to drain it. Turn all of the fixtures off and then turn the water back on. Open the fixtures one at a time starting with the lowest in elevation and proceeding to the highest. It seems to work for me.
I grew up in San Antonio, TX. and the water there is some of the 'hardest' water anywhere. The water source for all of south Texas is a limestone aquifer and limestone scale tends to foul up showerheads and faucets within 10 years or so. I lived in one house for 10+ years and never had any problems with copper pipes. Showerheads and faucets are another story.