How can I work legally with an I-485 EAD and a pending OPT EAD?

What you were advised is very wise. You should continue your F-1 status under the OPT program. Yes. You can work with your I-485 EAD in the meanwhile.

You can work with I-485 EAD.

Unless I am missing something, I am not sure why you would file for an OPT EAD when you already have an EAD based on your pending I-485. It would not only be duplicative but money and time wasted in filing for another EAD. Also, not even sure you would be approved for an OPT-based EAD when you are no longer maintaining non-immigrant status per se (since you have a pending adjustment of status to permanent residence).

Yes, you should apply for the OPT EAD. But note that one has restrictions as to the number of days of unemployment and terms of employment, whereas the adjustment based on has no restrictions. So if you sure you will not have unemployment in excess of 90 days, you can apply for the OPT one in addition and work on the adjustment pending one meanwhile. I think your advisor is smart; it is good to have a fall back position.

You are permitted to work legally with the EAD received from filing the I-485. You are not required to apply for OPT.

Yes, you can.

The I-485 EAD is good enough and it is even better than OPT EAD for several reasons. You can work with it and you do not need OPT EAD.

You can apply for the OPT (I-20 endorsement), but indicate you have the work card through the c9 application so do not need to file and pay for the I-765.

I would advise you to work with the OPT EAD instead of I-485 EAD so that you could maintain your current status until you get your conditional green card. If you work under the I-485 EAD, you are giving up on your F-1 status as of that time and if for any reason your I-485 is denied, you do not have another status to fall back on.

You no longer need OPT or F-1. You can work on I-485 EAD.