Mac's Motivation for working with the Soviets
Unlike most of the villains of the Indy (movie) series, Mac's motivation for working with the Russians seems to be unclear.
I mean for example--Belloq was probably contracted to work for the Nazis given his reputation (and if you take the novels into account had an obligation to the Nazis after working for them previously in 1933). He and Indy had a history of being rivals and Indy despised Belloq for his treacherous and parasite-like nature--reaping the benefits of his and others' work and leaving the real workers to die while he claimed the fruits of their labor. As Belloq said in the film, the Nazis weren't his friends--he looked them essentially as tools to further his own personal goal. He didn't intend to share the Ark with Hitler, at least, not until he was finished with it. He might not have been on the trail for the Ark until the Nazis contracted him; we can't and don't know. It might've been an interest of his..but we're never told for sure, and the tone of the film suggests Belloq is under their employ, but acting as ''project leader'' because of his knowledge.
Elsa was a Nazi probably because she was Austrian and maybe felt buddying up with the national government, and also using everyone--Henry, Sr., Donovan, Indy, the Nazis in general--was the best way to achieve her 'prize' of the Grail. Donovan had the same motivations as Belloq, and made it clear he had no intention of letting Hitler take the Grail--He was using the Nazis as tools to get what he wants (though why the Nazis worked with Donovan, who didn't seem to be an adventurer like Indy or ''obtainer of rare antiquities" like Belloq but instead simply a rich hoarder, is unclear).
Mac on the other hand, being a friend of Indy and going on plenty of adventures with him, had no reason to work with the Nazis and betray Indy, potentially to his death. If he worked with Indy for nearly 20 years (The Ultimate Guide has their first meeting as 1939), and they had seen a number of weird things (for example the Zombis in the Army of the Dead novel) why didn't he just propose he and Indy search for Akator? I mean from knowing Indy for so long I'm sure he was aware how capable Indy was as an adventurer. I understand why the Soviets wanted the Skull but not so much why Mac felt he needed to work with them to get it.
I don't see why he felt he needed the Russians or needed to betray and try to get Indy killed to get to Akator. Sure, like Elsa, he was playing everyone, but still...If one of your closest friends, long time friends is a world famous adventurer/''grave robber'' who you've personally been on adventures with, why would not just convince him to go on ''one last adventure'' for Akator? It makes a lot more sense to me anyway. If you bring Russians into the picture, they're not going to let him keep all the 'gold' at Akator, which seems to be what he was after; If it was just him and Indy searching for it, it'd be much more likely both would come home with a nice amount of treasure/cash.