If you don’t like flatpaks, then read on for details that may be helpful to you.
First, you probably don’t need to use the i386 version. If your system is x64, you can use that version instead and avoid the need for multiarchitecture support.
Second, as you have discovered, there are some dependencies that cannot be met by the 20.04 repository contents, but which can be satisfied via manual installation of packages from the 18.04 repository. See Gnome-inform7 6L38/6M62 IDE now running on modern Ubuntu & Fedora OS for the gnome-inform7 download I used (put together by user @interactivefiction). Post #74 in that thread lists the specific packages that must be pre-installed to satisfy gnome-inform7 requirements and how to get them.