Yes you need to be a supporter to use this. It is quite advanced and took some effort to develop. And we need to add more premium features to increase subscription rates.
Yes that will be added. CSV was done first because that is more accessible to non-developers and this is tabular data.
Intervals.icu uses the info from the session message in the fit file if it can to match the device. Otherwise the change from one point to the next must be at least 1.2m to count towards gain/loss.