Intro
I left Ahmadiyya mainly because of what I read about the Lahori-Qadiani split in the Ahmadiyya movement.  I was disgusted by the behaviors of the Mirza family.  In my research, I found an old debate by these two groups in 1915 and published in 1916.  In this debate each party put forward its arguments before an impartial, non-Ahmadi Muslim arbitrator as to its interpretation of the claims of  Mirza Ghulam Ahmad. The arbitrator’s verdict, printed in this book, is presented to show that he ruled that the Qadiani explanation of his claim, that he claimed to be a prophet, was correct.

The debate
You can find it here also: http://wiki.qern.org/ahmadiyya/publications/mubahitha-shimla

This debate is the definitive reference on what the Qadiani group of the Ahmadiyya think about Mirza Ghulam Ahmad as a prophet, and how the Lahori groups deflect the issue.

This debate on the innovative and complicated theology of the Ahmadiyya groups was conducted at a conference held in Shimla that was called by the Ahmadis of Shimla.  Shimla being a rich resort, these Ahmadis invited both parties and appointed Shaikh Muhammad Umar BA, LLB, as the arbitrator.

The debate took place on 23rd December 1915 and was published in Qadian as Qaul Faisal, Mubahatha Shimla (8.5MB PDF) on 4 June 1916. These proceedings represents the closest we will get to the original positions and it includes many very interesting references and interpretations.
The arbitrator concluded:
  1. Mirza Sahib is not a permanent (i.e. direct) prophet, nor is he a real prophet in the sense related to being legislative, but he is a indirect non-legislative prophet.
  2. According to Mirza Sahib, he is a real prophet in the sense that he attained the office of prophet via Muhammad (saw).
  3. According to Mirza Sahib, no other person has been bestowed this office in 1300 years Hijri.
  4. According to Mirza Sahib, he is entered into the group of prophets and between him and past prophets, the only difference is the means by which he became prophet, and not in the essence of being a prophet.
  5. According to Mirza Sahib, as he himself is a prophet, he has superiority, with all aspects of glory, over Jesus (as).
Urdu original:
Mubahatha Shimla Conclusion

Lahori Defense

Although the Lahori group has never been vocal about this debate, after this article was published, Dr. Zahid Aziz has written a defense of the verdict against the Lahori group on the Lahore Ahmadiyya Web site, available at http://www.ahmadiyya.org/qadis/shimla-debate.htm.  None of the three points he raises — deferred matter of takfeer, Qadiani debater becoming Lahori, and Qadianis retracting their beliefs — is a rebuttal.  As we wrote above about the Lahori position, the article is indeed a deflection.
Dr. Aziz’s point is valid that the Lahori doctrine has been consistent.  This is so because it was explicitly engineered by M. Ali and K. Kamaluddin as to not offend Muslims and try to stay within the realm of Islam.  During the British raj, the Qadianis went full-bore with Mirza Ghulam Ahmad’s claims of being a prophet, and then they have had to retract or back-pedal their views in the face of Muslim opprobrium.
There are two distinct phases of Mirza’s prophetic career: before 1901 and after 1901.  Right in that year when the Mirza staked his claim to being a prophet, two of his stalwart followers, Ahsan Amrohi and Abdul Karim Sialkoti publicly argued about the matter.  Hence, the Lahori view that there was no change at all is contrived.  And that is exactly the point that makes this debate, held within a year of the Split, so important.

The Debate
Mubahatha-Shimla