So here's what I hope will be an open discussion about how the Launcher installs content. So far, here's what I've gathered. I'm hoping this discussion will lead to more information about how and where exactly the Launcher sticks stuff so that we'll be able to bypass it entirely at some point.
Sims:Primary Location: Installed to SavedSims folder, extension changed to .sim.
Other Locations: Sims3Pack copied to Downloads folder, a copy of the package kept in the DCBackup folder.
Uninstallation Action: I didn't write it down at the time... Oops.

As I recall, only the Sims3Pack file is deleted. (Please verify, someone.)
Verified Workaround: Just extract the package and stick it in the SavedSims folder. Keep extension as .sim.
Lots and Households:Primary Location: Installed to Library folder, file named as ebf_CRC_OF_FILE.package.
Other Locations: Sims3Pack copied to Downloads folder, a copy of the package kept in the DCBackup folder.
Uninstallation Action: Again, I didn't write it down at the time. As I recall, only the Sims3Pack file is deleted. (Please verify, someone.)
Verified Workaround: Just extract the package and stick it in the Library folder. Filename doesn't matter. .package extension.
Patterns and Presets:Primary Location: Patterns and presets are merged together into a dbcdb0.dbc file in the DCCache folder. Instance IDs are changed and an index file of some sort keeps track of them.
Other Locations: Sims3Pack copied to Downloads folder, a copy of the package kept in the DCBackup folder. The game also seems to stick a copy into compositorCache.package OR simCompositorCache.package, as well as possibly in the userPresets.package file. (Not sure about that last bit...)
Uninstallation Action: Launcher removes Sims3Pack from downloads. That's it. Due to also being in DCCache and one of the above mentioned packages, it is nearly impossible to delete.
Verified Workaround: Patterns will appear if placed in the Mods/Packages directory, but will not appear as custom content. I have not yet found a way to import presets sans-launcher. If the format of the index in the dbcdb0.dbc file could be figured out, I could possibly have the 3Viewer stick content into there, but that would be a last ditch effort.
Objects:Primary Location: I haven't seen objects installed by the launcher yet. Any info appreciated!
Other Locations: N/A
Uninstallation Action: N/A
Verified Workaround: N/A
CAS Parts:Primary Location: I haven't seen CAS Parts installed by the launcher yet. Any info appreciated!
Other Locations: N/A
Uninstallation Action: N/A
Verified Workaround: N/A
Neighborhoods:Primary Location: I think I heard these are Sims3Pack files... I haven't seen it yet. Any info would be appreciated!
Other Locations: N/A
Uninstallation Action: N/A
Verified Workaround: N/A