"Can you address how new instrument pilots should handle clearance changes in busy airspace, single pilot, IMC and when pilot workload is at the max?" - Tom M.
“First, always remember that you are Pilot in Command and responsible for the operation of your aircraft. If you are getting in a situation where the workload is higher than you can safely handle, you need to take steps to manage the workload.
If ATC calls with a reroute and you are unable to copy, ask the controller to Standby. Or ask for an initial heading to fly until you can copy the route and get it loaded in the GPS.
Preparation and practice is the best way to avoid getting task overloaded. Review the route before takeoff. Get all the charts organized. If you use an iPad or other electronic device – practice until you are proficient with it.
For the average GA pilot, an autopilot is required equipment for single pilot IFR. I would not think of flying single pilot IFR without it in a busy area.
Saying Unable in a busy Metro area will not endear you to the controller, but if just say Standby a minute, let me get organized and I’ll be ready to copy – they will work with you.”