The jet air inlet is probably not connected to the blower. Being so close to the spa, the jets need a blower. The only reason to use a blower is because the jets or bubblers in this case, cannot get enough air with a passive air pipe.
One of the reasons that the air pipe squirts water is because the valve cap is broken. If you had the threaded cap installed, this should prevent most of the water from jumping out. But because the pool deck is above water level, there should not be any water pouring out of that valve, correct? When the pump first turns on, there may be a little water than jumps out but once the venturi's get going, they should be sucking air through that port. If after some time, there is still water coming out of the air port, then you may be right in that there is some debris in the air line which is clogging the jets. Given the broken pipe and the location of the air inlet, this is a pretty good possibility. One thing to try is to remove all the jet nozzles (deep inside the spa wall) and run the jets to see if you can flush out the debris.