The name is spelled Feliksa here.
Which doesn't prove that Felikska wasn't a diminutive of Feliksa. One cousin used the given name and the other cousin used the diminutive in the obit. I am curious how a Silesian or German-Polish person might consider the name.
Here we have Felicska and Felikska.
Perhaps Felikska is a misspelling of Felicska? Of course, the s is redundant in Polish here...