I don't know which saved games you have downloaded but the saved games of Louis can be downloaded from his website: www.pcgameswalkthroughs.nl
Go to the English section and click on Lighthouse the dark being.
At the top of the walkthrough you see: My Lighthouse Saved games.
Download these files, unzip them and place them in the save file folder of Lighthouse, then start the game.
You wrote in your reply that you downloaded from another site and we don't know if these saves works with ScummV.
There was another thread about the walkthrough of Lighthouse the dark being in march 2018.
Some Gameboomers have played the game and have used the saved games of Louis.
The saved games of Louis are made with the GOG version of the game and so you have to use the proper saved games.
go to the website of Louis and click on his name in blue, then you can send an email to Louis and ask him for help.