Hi yaqoubdesign,
@yaqoubdesign wrote:
1) I managed to get the 3d step file associated with the part in my
managed library. But- now ERC gives errror: board and schematic are not
consistent "part IC1 has inconsistent packages".
What is the best way to update the schematic / PCB properly ?
After mapping the 3D step model in the manage library save the library. Return to your design and click on Library/Update and select the library. Another option, in the schematic editor use the REPLACE command and replace the component in the schematic. Repeat the same step in the Board Editor. This is not intended to be this way, by doing the design update with the edited library should update all parts in the design. Make sure that at all time you have the board and schematic open.
Just for good practice, run ERC before updating the design with the new library.
2) I see in the existing default libraries there are some 3d models
available. Can a 3D package be cross-used for other parts?
Yes, it is possible. From the EAGLE Library editor, click on the "Import 3D Package" and select the option Import Local 3D Package. From the Import Package Dialog box you can navigate our the USED libraries to find the package that has the associated 3D model.

3) Assuming the answer to (2) is yes, is there a way to search the
libraries for 3d packages- just to see what is available already?
At this time that is not available, by expanding the libraries, you will notice at 3D symbol next to the package that has been appropriately mapped.
4) From an outside perspective, this whole approach of including a 3D model
is very awkward. We have library services like SamacSys which gives all
the information needed in one-go. Just search, download and done-
INCLUDING the 3D package. Is all of this just a temporary work around
for a future update that will be much more intuitive? I hope so.
We are continuously making improvements to make this flow easier. Consider using the Package Generator in a future project. The package generator builds the EAGLE package and associated 3D model in the same step. Now you can use the replace command in the SAMACSYS library device editor to replace the package. 
5) It seems strange that so many parts in the default library do not have
3d models, when they are readily available. Do you know why this is?
EAGLE is been around for a very long time, possibly over 30 years. All those years we have collected a massive amount of libraries which have been created by the original CadSoft Team, others are contributions by the community. We have a team which are continuously updating the libraries. That is the convenience of having manage libraries, because you can updated them as they become available directly in EAGLE Library Manager.
We greatly appreciate your participation on the forums. I hope you stick around and provide us more information about your progress with EAGLE.
Have a GREAT DAY!!
Best Regards,
Ed
Edwin Robledo
Tech Marketing Manager