How do an OPT EAD and a I-485 EAD conflict with each other?

These are two separate EADs based on a different immigration application/benefit. If you are going to have a gap in EAD validity, you may not work without EAD and need to wait to receive an I-485-based EAD to resume employment.

There is no technical conflict between the I-485 and OPT. However, the filing of the I-485 is conclusive proof of the fact that you wish to remain in America permanently. Therefore, USCIS could argue that you are not entitled to a renewal of the OPT. This could also be argued by USCIS if you already have an an EB-5 case filed or approved. This does not happen in every case, but I have heard of it happening. Maybe you can apply for the EAD extension before you file the I-485. There is no guarantee that this will work. Finally, you should get work permission some time after you file the I-485. It was taking 3 months, but recently it is taking longer.

Your EAD based upon your optional practical training (OPT) and your EAD based upon your application for adjustment of status on Form I-485 are two separate and independent matters. Thus, if your OPT EAD expires, then you do not have any work authorization until your new EAD based on your application for adjustment of status is approved. If you worked after the expiration of the OPT EAD, you would be deemed to have engaged in unauthorized employment and same could adversely affect your eligibility to apply for an obtain approval of your application for adjustment of status.

There is no conflict per se in working under the OPT/EAD while you file for the I-485/EAD; however, once you file for the I-485-based EAD application, you will no longer be eligible for the OPT/EAD extension since you are no longer maintaining non-immigrant status.

The two are mutually exclusive. They do not conflict. Also, there is nothing wrong if the EAD overlaps.

They will not let you have two at the same time if they know about it, so just apply for the renewal though the I-485 when it is time. Apply six months prior to expiration.

You may not work without EAD. If there is a gap between OPT EAD expiration and AOS EAD being effective, you must not work during that gap period. However, if I-485 were filed before your OPT ends, then you are allowed to stay in the U.S. under the AOS pending status. Make sure that your lawyer carefully reviews the pertinent dates and guides you so that you are not out of status or do things that will violate your status. With the Trump administration looking for any reason to deny cases, this kind of thing could trip up people if they do not pay careful attention to details.