Hi,
thank you for your questions. See below my comments:
1. Rappid BL and OpenSDA (drag/drop loading) are two different loaders
Rappid BL firmware (.rbf file) resists on the target MCU (S32K144) and it is executed after reset.
OpenSDA firmware (.sda file)is located in a different chip(OpenSDA MCU) and behaves as USB <-> jtag converter (kind of "lite" version of PE Multilink integrated on the board)
https://www.nxp.com/document/guide/get-started-with-the-s32k144evb:NGS-S32K144EVB
RAppid BL advantage is you can program the target mcu over CAN/UART but it requires to be pre-programmed on the chip. OpenSDA is in fact "independent" Jtag debug probe.
2. to program a file to the S32K mcu over RappidBL you need just usb cable (UART mode) or Vector CANCase/Card XL (CAN Mode) converter.
3. Com Port is only the selector of a correct COM port to be communicated with the target over UART/USB. When USB cabel is plugged into S32K1xx board a new Com port should appear on your machine (could be checked in the device manager) and you should select this port.
4. No additional PE multilink required. You can program bootloader by OpenSDA (drag/drop). Anyway if you'd need fo some reason to program mcu from P&E external probe you can use J14 SWD connector.
The other one (SDA) is for updating firmware/bootloader in OpenSDA chip - I'd recommend you not to use OpenSDA JTAG connector untill you fully understand what you do.
Hope it helps.
Stan