So, since only Balamb trains people in the usage of GFs- remember, Selphie was an special case- you propose that they only draw and cast, a process that requires the usage of a Junctioned GF.

So, you aren't actually offering a theory. In fact, you are offering the opposite of one, since you are blatantly contradicting what is a known element of the game, since as I mentioned already, Galbadian soldiers- and your party themselves- can hold stocked magic without the aid of GFs, and attempting to say that instead of having a machine which allows for limited emulation of GF ability, soldiers and human enemies can draw and cast magic 'somehow'.

Color me unconvinced.

And as I recall, no one was all that surprised at the existence of the a junction machine. Just that it emulated Ellone's powers.

And no. Selphie was not a Trabian SeeD. She became a SeeD after passing the test AT BALAMB. And Irvine was not a SeeD. Ever.