Running foot pod

The first one is that the vector needs to be in earth coordinats

I don't think so actually - if you know when the foot has made contact with the ground, you literally just take the length of the 3D vector between the current and last time it hit the ground - you don't really care what direction it 'thinks' it is?

is there a simple app to stream data from the acc/gyro/mag to an android phone?

From the Bangle? I think https://banglejs.com/apps/?id=sensible will do it (advertise the Bangle's sensors), although i'm not sure how fast.

You've got 8MB of flash - I'd expect that even recording at 100Hz it'd easily last you for the length of a run as long as you store the data in a reasonably compact form?