Combat AI target selection should prefer closer targets
If you're fighting one or multiple skeleton archers they always will target your character and won't pay attention to guards or player summons attacking them, making them unable to defend themselves against attacks from a third party, if they by any chance pay attention to your summons first (when placed by the console) then simply make your character stand in their line of sight and they will be back to always attacking your character regardless of any third party there is, note that imperial archers (NPCs) don't seem to be affected by this but needs confirmation.
In morrowind they do defend themselves against third party attackers and might shift focus to the player character if the PC got closer to them, but will immediately shift focus to third party attackers if the player character is at a distance from them.
The conclusion is skeleton archers primary focus is the player character at almost all times.
OpenMW: https://streamable.com/l3bhhb
My Save: Summons_VS_Archers.omwsave