It would be great for single-player maps if there was a way to transfer artifacts from one hero's inventory directly to another hero's inventory.

This has been working for a long time, and not just in single-player.
[Shift]+click on an artifact in one hero's inventory/backpack will place the artifact in another hero's backpack (also works in the open inventory window).
[Shift]+one-way artifact transfer button - transfers all artifacts from one hero's backpack to another hero's backpack.

Also:
[Ctrl]+click on an artifact in one hero's inventory/backpack will equip the artifact on another hero (also works in the open inventory window).

[Alt]+click removes the artifact to the backpack / equips it on the hero (also works in the hero window).