Can Airflow issues caused by a dirty/clogged A Frame Coil make it hard to reach set temp & cause flow probs in some vents?

Yes a clogged coil will not properly coil or give you good air flow. If your gonna clean it your self use a soft wire brush only move brush in direction the fins run on the coil don't bend them flat. Do not use 409, lysol, mean green ect. Use ether plain water or cool cleaner. You'd be best served to have have a hvac company come service it. They'll have the tools an knowledge to fix it.

Yes.

Sure it can and could cause the coil to ice up in the cooling mode. This should not happen if you have a proper filter system.Dont know if you have a high efficiency furnace but most of those also have a secondary heat exchanger coil and can plug with dirt. It would likely plug up first since it is the first thing the furnace blower blows through.