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
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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!