It seems to me that Max’s ‘situation’ of him dying from radiation poisoning (I agree that it’s the trigger) is causing problems for him, which is what motivates him to get to Elysium at all costs (what @Writegeist said), even brushing aside Frey’s request for help, and later offering up the information in his head to Kruger, even after Spider had told him that the data in his head could save everyone (including Frey’s daughter).
As for Spider, I think he is an objective character, namely the protagonist, as he is the one spearheading (prime mover) what I believe is the OS storyline of ‘opening the borders between earth and Elysium’ (problematic Activities), which he admits himself in one scene. There is even a scene of him trying to do this early on in the story before him and Max even meet, which means it is a concern that involves everyone, not just him and Max, which is why I don’t think he is the IC. On the other hand, it is Frey’s presence (being captured by Kruger) which trigger’s Max’s eventual change of heart, which tells me that the emotional connection (heart of the story) is (supposed to be) between Max and Frey. After all, Spider does make an appeal to him early on, but it is his relationship with Frey, not Spider, which tips the scales.
I think the reason why Spider has so much screen time is that Blompkamp made a mistake, which is why what was supposed to be the subjective throughline (max and Frey) came up so half baked, or more like nonexistent, and implied more than anything, through the flashbacks and all. Which would explain why the heart of the story feels nonexistent, and Max’s ‘character’ development rings false.
Sorry if I’ve gotten too far ahead and talked about too many points here. Just let me know, and I’ll edit my post accordingly.