Thanks for this. It clears up which batteries actually manned guns in the rest of III Corps and that there was no third battery with Demont.
Songis lists at date of Aspern for Divisions 1-3 of III Corps are identical to the 15th June list except that Divisions 1-3 have 2 more 8pdrs each and no 12pdrs. There is also 1x12pdr listed in the park. This therefore implies that 14/8 & 18/8 when engaged at Aspern had 7x12pdrs, plus 3x8pdrs and 3x4pdrs, assuming they had not 'borrowed' any guns from II Corps whilst Demont was attached to it.
Napoleons order of 24th May for the concentrated 12pdrs with Demont to be broken up certainly triggered 6x12pdrs in 14/8 and 18/8 to be swapped with 2x8pdrs each from Divisions 1-3.
Remains open whether this 12pdr concentration was just done for Aspern or happened earlier in the campaign - but as you say only a rummage through Vincennes might answer that.
What all this does seem to show is that whilst 12pdrs were clearly by policy not normally concentrated in battery as a Corps asset at this time, they could be as a tactical expedient.