What is the best way to simply get access to all airline seating information? For starters, just:
It’s nearly impossible to get access to all airline seating info as some airlines may not share this data in the first place.
However, there are two main options. The first one is obvious: you may contact airlines directly and ask for their API access with seating capabilities. For instance, Lufthansa Open API provides seat maps. But most airlines don’t support APIs at all.
The second option is to source seating info from GDSs.
They still may be limited by the data that carriers provide.
It doesn’t look like SeatGuru has an open API, but it’s also worth trying to contact them directly.
Getting access to GDS APIs isn’t that simple, but it seems like the best option for your problem.