[rengerts]

Clearly some forms of istighatha are allowed depending on the niyyah and belief behind the call; and some are shirk/haram, again depending on the underlying niyyah and belief of the caller.

you are right. i don't see how any of the deobandi brothers can argue with this. this is their belief. i think when first three are involved that some barelvi brothers do not draw the line. not all of them though.
however i must agree what mufti taqi says.
we say ya ayyihun nabiyyo assalamu alaika in attahiyat with the belief that the Holy Prophet (PBUH) is conveyed greetings by the angels on our behalf. that part is unanimously agreed.
i personally wont do 'salah' al ghauthiyyah because
a) not done by the Prophet (PBUH)
b) not done by first three generations of Islam
c) no concrete proof of authenticity.

but thats my personal opinion.