I would look at the coil to see if it was freezing up. You say it's only 10 MO old . I would call the AC Co back for a warranty check.
55 degrees to 60 degrees
You need to call a qualified heating contractor for service.
64
There is no set temperature that the central air should be set to. Most people choose to set this temperature to about 70 degrees Fahrenheit.
straight from the vent should be around 47 degrees
It should be a 4 ton.
There should be at least a 12 degree split between the incomming air and the air comming out of the vents.
Should be 2.5 ton
The air on the outlet side of the air handler should be approximately 18 degrees colder than the air on the inlet side.
The filter may be too dirty and blocking air flow. It should be checked for cleanliness once a month. Also, there could be a leak in the system.
I should think that the lowest it ever goes in Africa is about 15-17 degrees during the day
Ultimately, it should be whatever temperature makes you comfortable, but I'd say a normal temperature would be around 65 degrees or so.