First note here is to not to mix AVD with ConfigMgr and Intune. AVD is a VDI-like infrastructure hosted in Azure. Intune and ConfigMgr are management tools. You can use either ConfigMgr or Intune to manage AVD systems thus AVD is out of place for this discussion.
- That depends on what you mean by "package". If you mean, can you use any valid Windows Installer in Intune and ConfigMgr then the answer is yes. How you get them into both is different though. Alternatively, if you are referring to re-packaging existing installers into another installer technology, that's a very bad idea in general. You should always use the installer provided by the vendor when possible.
- There are folks that have scripted out solutions for migrating, but I don't think there's much of a valid use case for creating the same "packages" in both generally.
- MSIX is a standard Windows installation technology that both Intune and ConfigMgr support natively.
- Assuming you mean re-packaged, then that's a loaded question. As noted in #1, you shouldn't ever want to do this in the first place, however, technically, sure, any installer can be re-packaged as an MSIX.