In a Lightning Experience app, in a list view, can I add a custom action button/link to the action menu (highlighted below):
This button/link will provide a sort of checkout mechanism that changes the owner id from the object to the current user, then opens a custom LWC for them to edit details related to the item.
Alternatively, a separate link/button in its own column (not underneath the Actions dropdown menu) would be even better.
Best Answer
You cannot customize the list view item's actions. Actions in Lightning Experience mention this:
There's a couple lowly voted ideas asking for this ability:
You can customize the buttons that display on the list view itself. This would allow you to select one (or multiple) records from the list view and perform an update action on them (quick action) or pass the record ids to a flow (through URL button) or VF page.
Update Record
.Setup --> Object Manager --> Your Object --> Search layouts for Salesforce Classic --> Edit "List View" layout
If you need more complex logic to be initiated by the button (multiple screens) or to leverage a LWC, you'd have to look at utilizing it within a Flow or a VF page in a List Button that has
Display Checkboxes (for Multi-Record Selection
enabled.