Under the Options menu in the iFrame Card, there is a setting "Allow Scrolling" which gives you the option "no" (scrolling is not allowed) or "auto" (scroll bars show up when necessary.) We don't have "yes" because it's very similar in functionality to auto, but it will show scrollbars, regardless of whether scrolling can be performed or not.
If "auto" is not working for you, it might be a browser issue. Technically, the iframe scrolling attribute is deprecated. However, like you, we use it, because it's more consistently effective across browsers than the modern CSS-only solution. Another thing to check is that whatever content you are iframing into the card is also scrollable at the content source. It might seem like a silly suggestion, but I've encountered it at least once.
Unfortunately, you are not going to be able to use the Text Card for iframes, as what you are seeing rendered is actually a bug in that it renders at all; more recent versions do convert iframe tags to text, meaning it wouldn't even come across as code.