Hello and welcome.
It does sound like you are still getting air in the cooling system. The problem of no or little heat at idle is a result of air pockets. When the engine is at a higher RPM, more coolant is flowing, so more heat.
The low coolant because of a leaking intake manifold gasket is a VERY common problem, but sometimes the shop overlooks the fact that there may be additional problems, namely- the rear head gasket.
When they fail, it give the exact problem you are talking about.
so why is the heat to the rear fine?
For some reason the air pockets never make it back to the rear heater core. I believe it has to do with the way the hoses are run, the air always gets into the front core.
You may be able to see air bubbles in the coolant with the cap off and running at idle. Removing the passenger side rear spark plug may show signs of burning coolant.