I don' understand what it means/adds on these sentences, since there's an adverb before them and the particle can be omitted and it still makes sense
This usage of 「は」 is for emphasizing the meaning of the preceding word. It often implies a contrast as well.
"I am not doing it now. (I might do it later on.)"
"If a person became this weak before your very eyes, anyone would be a little bit more (if not by much) gentle."
「弱られたら」 is in the famous "suffering passive" form, meaning I had to use a different grammar around the verb in my translation to make any sense in English.
This implies "I won't do (something) now, but I might later." Without the は, there's no implying the "but I might later" part.
To me this implies "will become a little bit kinder, but not necessarily a lot kinder, but definitely not unchanged or reduced in kindness level either". Without the は, there's no implying the "but not necessarily a lot kinder, but definitely not unchanged in kindness level either" part.
Open to feedback!