First of all, I am very happy with this platform!
I am using a Garmin Forerunner 255 and a Running Dynamics Pod. The cadence on Stava and Garmin Mobile Connect is correct (around 180 spm when running). Intervals displays the number 90, so half the spm.
Strava
In the meantime, I have a custom activity field and a custom interval field. The only problem is the lack of the graphic, but for me it is ok with the total and interval average.
Tx. You can do it a little easier using activity.average_cadence * 2 and interval.average_cadence * 2.
Also with your interval code you need to use interval.start_index and interval.end_index for this sort of thing. The times will only work if there is 1 second per tick and no missing data points.