How to email professor with group concerns about class?
I think e-mailing him essentially what you wrote here is fine. You are respectful and understanding of everyone involved, and have legitimate concerns. I think an e-mail would be very likely to get a favorable response.

I think it is fine to e-mail from your institutional account, but if you prefer anonymity I think it is also fine to sign up for a Gmail or Yahoo account or something and e-mail from that.

Probably best not to mention names.