Catherine seems to me to be a brave but psychologically conflicted character, akin to the narrator's wife in Garden of Eden. There is a moment in Farewell where Catherine offers a glimpse into the darkness an emotionally disordered mind, which does not make her submissive and weak, but rather makes her a natural counter-part to the desperate and aggravated Frederic.
The female characters from From Whom the Bell Tolls are also clearly not submissive, but are soldiers risking their lives in the mountains of Spain.
Most modernist novels are existentialist and are characterised by the fragmentation of the identity of a single character.
Hemingway also reveals to us different facets of the character Catherine:
1. At the end of Ch.6 Frederic tells Rinaldi that he and Catherine are "friends."
2. At the end of Ch. 14 they are lovers.
3. In Ch. 18 they are virtually husband and wife:Catherine says:"We're really married. I couldn't be any more married."
4. In Ch. 23 Catherine feels like a whore : "But it isn't nice to feel like one (a whore)."
5. At the end of Ch.23 Frederic quotes two lines from Marvell's "To His Coy Mistress." Did he regard Catherine only as his mistress?
6. The novel ends with Catherine's death after she had given birth to a still born child.
So Catherine is a character with multiple facets to her personality and it is impossible to comprehend her fully: she is friend, lover, wife, whore, mistress and mother of a still born child.
She is certainly not completely submissive, especially in the initial stages of their relationship: In Ch.5 she slaps Frederic.
Instead of trying to find out whether she is weak or strong or somewhere in between it would be more profitable trying to figure out whether there are more hidden aspects to her personality.