Web- and PW-Portal: Login to portal using email address?

I don't see any options that would allow you to do that. Here is the guide on the available authorization methods:

support.oneidentity.com/.../authorization-and-authentication-guide

Actually I see this: Decentralized identity and Decentralized Identity (rolebased)

I think this will accomplish what you need

Can your email provider act as an idP? If so, you could use oAuth. 

We're using it in our web portal to log on using our email address (Google based)

Yes you could do OAuth, I have a setup in my lab where I have an Azure tenant setup as my IDP, I use the users Azure email address. I have my users persons default email address populated with the exact same email address and then in Identity Manager for this IDP configuration I use that for my column to search on the Identity Manager side. Person - Default Email Address.

oAuth (Keycloak) is not the problem. It is possible and working fine. But logging into Password Portal, in case you don't know your password, is possible only with user name. Both possibilities (password questions or pass code) are authenticating only with Central Account.

I see. We're not there (angular) , yet. That needs development. You can query the email address then do a search for the centralaccount and use it in your call to the login process. See the passcode-login.component.ts in qer/src/lib/password/passcode-login, but I guess you already figured it out.

The only thing here is that you'll need to start a session to query the centralaccount. In our labs, we developed a custom api with fixed credentials that does the trick and we're working with it in our (painful) migration from web designer to angular.