Those are quite difficult questions. I would always recommend to test everything - to see not if it only works but also what is the performance.
I think, there are differences in number of IPUs between CPUs (Solo, Dual, QUAD). So by careful.
Here is picture from the QUAD datasheet (Page 456):
https://www.nxp.com/docs/en/referenc.../iMX6DQPRM.pdf What you may want to consider is maximum speed of the ports / maximum resolution. I am not expert for graphics, so I am not exactly sure what the capabilities would be
3) We use USB and UART. We do not use JTAG
4) USB will not provide enough power for QUAD CPU
5) PCIE is often used for Wifi card. Ethernet is very useful to access your board over network (not needed if you have wifi, but wifi may not be available in uBoot and updating firmware and debugging is much faster through ethernet). HDMI can be useful if you need to connect to the board locally (but not essential). USB is always useful (Bluetooth, keyboard, mouse, USB storage, camera, ...). SATA is useful for development (board works much faster and reliable from SATA than from SD), Audio (if you do not need it, it is not necessary). SD is useful (many boards boot up from SD, but SD is not very reliable)