Is there a way to show an Action button in the related list of an object in Ligning. The only way I see is to use the "New Button or link" with type "List" selected, but there we can not use the new Lightning action buttons which is not so good.
Only list buttons could be added as related list buttons.
And list buttons that are not JS are actually shown in the lightning related list.
See pictures.
Best Answer
Have you explored the https://github.com/jmdohn/Quick-Actions-Everywhere by Jeremiah Dohn.
This is a library for pushing quick actions everywhere in Salesforce (except for the global action bar, because, well, they are already everywhere!).