Origin of the Alans

[Merlkir]

While looking up an unrelated thing in John E. Hill's "Through the Jade Gate - China to Rome", I came across a chapter I haven't read through before - one discussing the origin of the Yuezhi. 
The book is from last year and he discusses various sources very thoroughly. I've not compared your hypothesis to his in detail, but he too feels the Gansu by itself would've been way too cramped.
Aside from an excellent translation of the Hou Hanshu, these two volumes also contain extremely extensive appendices, full of exactly this type of material. Really cool and they only cost like 20 USD each, since it's not a book published by a university, but rather self published through Amazon: 

https://www.amazon.com/Through-Jade-Gate...8QVNY8GZ31