Get some sort of thermometer that you can stick right at the vent. Measure the temperature of the air going into the unit, open the cold vent on the ceiling unit itself and measure the air temp coming out, then finally close that and measure the temp coming out your ceiling vents at each point.
You should see something like a 10-20 degree temperature differential between air going into the unit and air coming out of it. If not, you're probably right that you have a leak or something else going on. If you see the differential at the unit but not at the ducted ceiling vents, you may be picking up too much heat through your roof.
Also remember that with air conditioning airflow is key. Too little airflow and your coil may freeze because there's no air to give off its heat to the coil. Typical causes of that are dirty filters, closed vents or a dirty, clogged coil.
I am not an HVAC expert, so take everything I just wrote worth a grain of salt. I'm just sharing what I've picked up (right or wrong) over the years from when I've had to get various home and commercial A/C units repaired.
2015 Octane T32C
2012 Ford F-250 Lariat/FX4/Diesel