First off: Detachment D-99 was seconded to the Inquisition, in other words they borrowed it temporarily, it was not founded by the Inquisition simply used by them. As for the Exorcists, Deathwatch, and the Grey Knights? These chapters were also not founded by the Inquisition,  they work with the Inquisition but they are not part of the Inquisitorial chain of command and can tell an Inquisitor to go fuck himself if he wants.

The Deathwatch chapter was founded at the order of the Emperor before the Inquisition even existed, in fact the framework of the Inquisition was not even finished being laid yet.

Same for the Grey Knights, in fact the Grey Knights were ordered to be founded, again by the Emperor, again before the Inquisition was founded, and the original Grey Knights were in fact selected by Malcador the Sigilite and proceeded to train the recruits themselves.

The Exorcists Marine chapter is shrouded in mystery, yes the experiments were conducted by the Inquisition, however it was all done under the supervision of the Grey Knights, they could have just as easily founded it and no the Grey Knights do not answer to the Inquisition they work with them. Look it up on Lexicanum.

Second: Uh what? I draw your attention to: "That's cute. You still think the Inquisition is a bunch of perfectly straight defenders of the Imperium. Wait until you meet one who decides to execute an entire regiment because his project got out of hand, and then YOU having to be the one to do it."

Not to mention speculating on the abilities and efficiency of the IG is generally met with bolt shells. I would also draw attention to referring to a Commissar as a hangman.

All of that is heretical and people have been executed for less.

Combining the second and third points, working with xenos is heretical I don't know how I can possibly make this any clearer. Fighting alongside them for any reason under any circumstance is considered heretical, and no member of the IG or Inquisition in their right mind would admit to having done such a thing, regardless of the circumstances involved.

Finally; What happens on DakkaDakka or Warseer does not matter here. This is not DakkaDakka or Warseer. Perhaps not every bit of canon is 100% true, but there are certain things which are far more established than others, such as the fact that the Inquisition does not found battalions or chapters, simply requisitions or requests their services. Knowing the girl who did the Clan Pestilens character I can tell you it was specifically created for the sole purpose of finding out how long it would take for the account to be banned. Finally I would also note that I have yet to meet a 40k fan thinks that Matt Ward's continued existence is justified let alone anything he writes.