Waterway, the snap in inserts are probably long gone, if you look inside the jet you can see ridges that go around on the back of jet, the long legs on the insert when pushed inside would slide over the ridges and hold it in place, after awhile im sure the wear of the ridges, from taking inserts in and out, and the tension in the inserts legs becoming weaker would begin to result in the inserts being blown out of the jet constantly, then there just useless, and discarded etc, these insert type jets have usually been problematic in the field with spinner type jets being some of the worst.
Whenever you have a air blower on an air line the air blower can not be turned on without the pump running also, or the air will only do as you described and splash up on the deck etc
An air line without a blower will draw air through air line when pump is on only, this looks very odd to people when they see it with blower forcing air through air line
The pump can be on without the blower running, but the blower cannot be on without the pump due to the jets splash effect
I'm sure the jets still mix air and water without optional inserts but it may be possible to build up ridges on jet with something and see if a new insert would snap in, if that was even the reason behind current lack of inserts you might find out all you need is a new insert and it snaps right in,
If i could be there to put my eyes on it it would rule out or confirm the limited info i can derive from just a pic, or description you provided
