Hello Dbreton,
1) It sounds like your understanding is correct. When you select an IF bandwidth of 1MHz, we use a filter with cutoff points (6dB down) that have 1MHz banwdidth. This filter is also applied for the I/Q recordings.
2) (edit) See Justins response below. Also see this paper that lists various window functions and their ENBW/3/6dB bandwidths. See Eq. 11 for calculating ENBW.
https://www.utdallas.edu/~cpb021000/EE%204361/Great%20DSP%20Papers/Harris%20on%20Windows.pdf
We use the “blackman” window from this paper with the 1.73 bin ENBW.
Let me know if you have follow up questions.
Regards