It's not unusual for automotive ECUs to have 2 or even 3 microcontrollers in them. They are either redundant, or they work in some sort of lockstep scheme or there is a main controller doing all the processing and a smaller "safety controller" that is typically much more simple, but very reliable (think PIC16, 8051 derivatives and the like). I doubt though, that it's an ARM chip. IMO ARM controllers didn't really make it to the automotive industry (save for maybe some infotainment stuff). I've torn down some ECUs in the past and never saw an ARM processor or controller.
As for Infineon: they are very dedicated to automotive stuff, and generally pleasure to work with. And I can assure you that their microcontrollers have a fair share of the market. Maybe not in the uber-high volume stuff like airbags, but rather braking systems, stability, powertrain control, comfort modules etc. They also have some unique features like for example three core lockstep architecture. They also offer some other highly integrated chips like the PMIC in this module. For example one that has all communication interfaces and transceivers on a single chip.
One comment about the coating: what you seen on this airbag module is most definitely not brush-painted, but rather selectively coated. This is because you generally do not want conformal coating to be in between pcb and enclosure as it is relatively soft. When it deforms from applied forces, the pcb may start rattling in the enclosure. There are a few methods of selective coating, but two are dominant: cnc-driven spray nozzle or mask that shades some areas and large spray. To be honest the coating quality on that one is terrible (irregular, lots of air bubbles). It does not matter that the contacts are press-fitted or that there are programming pads. Coating is usually the last step before putting the pcb into the enclosure. There are many test pads that are coated so in-circuit test must be done before coating, and programming can be done at the same time. Here it can be different though, because of the capacitor holder thingie, which obviously was mounted after coating (there seems to be coating underneath it).
The EEPROM usually contains two types of info: first of all the parametrization data and checksums. Typically the OEM writes only a few software versions which are then parametrized using data stored in external eeprom/flash (for example vehicle mass, number of airbags, sensor angle and position etc). Crash data for analysis is sometimes stored in the flash, but some ECUs have non-erasable memory to prevent crash-code deletion (makes it harder to sell crashed and repaired car to unaware buyer).
Common mode choke is definitely for CAN bus, because from EMC compliance and EMI robustness point of view CAN one of the biggest interference sources inside the ECU and has to be heavily filtered and routed with extreme care on the pcb.
As for the airbag control algorithms, they can be quite complex. I can't say for sure how the software works, but I'd guess the following:
-tests airbag charges on startup, loads config data from eeprom
-gets the data from acceleration sensor
-gets the data about speed and if there is acceleration or deceleration going on
-senses the presence of passengers
-applies digital filtering to all the incoming signals
-calculates angle and force of impact taking into account speed and acceleration
-calculates inertia force which will affect the passengers
-determines whether to deploy airbag and is yes, than which airbags (for example only side curtains if it's a side impact)
and obviously a shitload of verification, checking, testing etc to rule out any false conditions like a road bump, driving over a curb, sudden engine stall etc. You don;t wan't an airbag to blow up in your face for no reason