1. Yes, you will need all IOM6 replacement modules for In-band.
2. & 3. If you retain the IOM3, I'm not sure you will achieve "Full Connectivity" without all shelves being cabled out-of-band, since it will detect different number of shelves between IBACP and OOBACP.
4. Regarding the firmware, so long as you are running 0190 or later (0191 latest) you only need to perform the upgrade once.
If possible I would try to go down the IOM6 replacement route. IOM3 has an End of Support date of 31-Jan-2021 (FAS8020 is not until 31-Dec-2022).
Support was added in 9.2P1 as a feature request under burt 1105827 for IOM3 for the FAS2500 and FAS8000 series only. It is not my understanding that this extended to IBACP support, which I as far as I'm aware have never existed.