In addition, the spell effects cannot be further enhanced with known magic, so one cannot use true seeing through a crystal ball or in conjunction with clairaudience/clairvoyance.
Actually seems to be saying that True Seeing can not be further enhanced by other spells.
I do not see how in any way it is saying that it functions separately from other vision based spells and functions as if they were not running (or vice versa).
Ok. Now let's say you have both True Seeing and See Invisibility active.
Do you see the Fog Cloud, the Human or the Dragon? Or all of the above?
You see the Fog. You may not even see the tell tell translucent of the invisible fog as the lucent and translucent would be taking up the exact same space. ie There is not blurred offset from the materials real self.
If the Invisible Polymorphed Dragon steps out of the Fog (which will be giving concealment and blocking line of sight due to True Seeing) then you would see a Dragon. The Dragon's lucent (True Seeing) and Translucent (See Invis) again are taking up the same space so likely no Translucent aspect would be seen.
So seems to me that the function of See Invisibility would be ruined by the True Seeing for things in the True Seeing's range. Let that invisible Dragon polymorphed into a human get 121ft away and then you will see the translucent human. Let him step back into an invisible fog effect and he will once again be hidden.
That all seems to be the plain functional interaction between those 3 spells and the Invisible spell metamagic.
That aside I'm sure most of us realize that True Seeing is a wonky spell that would be best redone and perhaps converted into a series of spells, maybe even something like Lesser, Normal, Greater versions that affect spells of 3 level ranges that Black Knight mentioned in a conversation this morning. But that is beyond the scope of this thread.Edited to give BK acknowledgment