Origin of the Alans

[Merlkir]

A) I've seen that hat section in the Loufan paper you mentioned some time ago. The paper itself is fine and interesting, but I find the identification of the hat a bit too specific. It could easily be a stylized depiction of the typical Phrygian cap, which is not exactly uncommon among the steppe people.

"Bronze figure of Saka period unearthed in Gongliu county, in the collection of the Ili Kazak Autonomous Prefecture Museum, Yining."
(the one to the right is in the museum of Urumqi)



Hat from one of the Tarim mummies:  


B) I'm still not sure how you determined the Northern tribes spoke an Indo-Iranian language. (when?)
If I remember Mallory et. Mair's "Tarim Mummies" correctly, their favoured hypothesis wasn't that the Yuezhi spoke Tocharian. Of course, the population would've spoken proto-Tocharian, a whole different language. (which easily bypasses the argument by time gap - that Tocharian is hundreds of years removed from the Yuezhi) We know the two (or more likely three) Tocharian languages evolved from another older language in a process that took quite a few centuries.  
MnM see pTch as the language of the original pastoralist settlers from the North, who were influnced by the oasis BMAC people and later by the lesser influx of Indo-Iranian people from the West. (in several waves, at least that's what linguistic loanwords seem to indigate)
While Afanasevo were originally favored as pToch speakers and therefore the ancestors of the Yuezhi, it seems the later culture, Andronovo is genetically closer to the Tarim mummies. 
Both would actually be quite well positioned to be the Northern people. (above Tian Shan, fairly close to your Altai) 
Now, if I get it right, you assume all the cultures we call Andronovo spoke and Indo-Iranian language? 
Looking at the situation now, and I'll admit it's quite late, so I might be missing something, it would be far easier to explain the presence of Tocharian by the eastern sub cultures of Andronovo adopting the language of the people they replaced there - the Afanasevo. (their relative isolated nature geography-wise would explain the isolated nature of Tocharian as a language)
I realize that by the now current dating of the two cultures, there's about a 400 year gap between them. Still, I have a hard time coming up with anything else to explain (proto)Tocharian in Central Asia. 

Simply sticking to Indo Iranian for the whole of Andronovo and subsequently the Yuezhi leaves us with unexplained presence of proto-Tocharian in the region. (especially since you consider the oasis towns to be part of a Yuezhi confederacy/empire)

edit: I should clarify that Mallory abandoned the Afanasevo -> pToch hypothesis by 2012: