Are there any I-526 based EAD work restrictions for an EB-5 investor?

I would maintain your H-1B status until you receive your EAD work permit. Once you have this in hand, there are no restrictions as to where you can work - it is an open work permit and you can also be self-employed. It is important to note that you will no longer be in H-1B status once you leave your current job and use your I-526 based EAD card and be in the U.S. in a period of authorized stay, often termed "I-485 pending status".

Yes, you can work on the EAD. The purpose of keeping a valid non-immigration status while I-485 is pending is to prevent you from going out of status just in case your I-485 is denied.

The EAD based on a pending adjustment is open market and unrestricted. If, however, this is a direct EB-5, you may need to be ready to prove your role, once approved.

There is no restriction on your EAD, although it is always recommended to maintain your H1B status just in case anything happens to your AOS application. You can start your own firm and work for yourself if you opt to use the EAD.