You can create a project offline. You have a few options: use the demo project, generate a configuration QR code without a server URL or fill in some random URL when configuring manually (https://foo.foo is fine, you just need a https:// prefix).
The fastest thing to do is tap "Demo project" on the bottom of the landing screen. If you're offline, you won't be able to pull the demo forms from the demo server but that doesn't matter: you now have a project that you can push forms into.
adb is a tool to push files between a computer and a device and yes, it does require connectivity to install. How did you previously push files to devices? You may be able to use that same strategy. Have you tried it? What happened? /sdcard/Android/data/org.odk.collect.android/files/projects/[somelonglistofnumbersorDEMO]/forms is the folder you'll need to put your form definitions into (documentation). adb can navigate to it and I believe some file manager apps can as well but I don't know of specific ones. It may depend on Android version.
You shouldn't need Briefcase for pushing form definitions though it can be helpful for managing submissions you pull off of a device while offline.
If you're going to be configuring multiple devices offline, you can use the QR code functionality. Once you've created a demo project, you can tap the D icon in the upper right and then the Settings button. I would recommend updating the Project display section to include information about your project. You may also want to do things like select which buttons are visible to users (Access control).
Then you can tap on Project management at the bottom of settings, Reconfigure with QR code, and tap on the QR Code tab to show the QR code for the current configuration. You can scan that into other devices to set up their projects with the same settings. Then you'll need to plug them into a computer and copy over the form definitions as you did with the first one.