Lecturer in a course I'm TA'ing refuses to give feedback on student assignment

No, you should not confront the lecturer again ("reopen") over this issue. That would not be an efficient use of your time.

You've talked to the course instructor. She has explained to you her justification. Presumably she's observed both cases of giving and not giving feedback for the final in the past (you have not). You do not have the power to compel her. You don't have other allies on the grading staff. You are ending your engagement there in the immediate future. You will not deal with this issue again. Move on.

Keep this in your list of "things I think I could improve on when I become a lecturer" for the future. Hopefully this will be a memorable case to experiment with later on your own. And you'll get to observe another institution's practices for comparison in the meantime. You may well be right, but you triply don't have the time to redirect this in your current position.

What the lecturer is doing is reprehensible, but there is nothing you can do about it.

A vast amount of educational research demonstrates that prompt feedback improves learning. The lecturer appears to be systematically avoiding giving any feedback, which is absurd.

Why would the professor do such a thing? You list her stated reason and one suspected reason. Neither of these is a valid reason for her to abdicate her responsibility to run the course in such a way that students receive feedback. Another quite likely reason is simply that she's lazy. The bare minimum she has to do in this course is show up to the lectures. Anything else added to the structure of the course will increase the amount of time she has to spend. If she assigns the students graded work, then even if she doesn't have to read the work herself, there is some nonzero amount of time she has to spend handling that, even if it just means entering scores into a spreadsheet or something.

Her justification for this is that she do not want students to pass the feedback to the next class that will take this course and thus avoid copying of answers.

There is a very simple solution to this problem, which is that she needs to stop reusing the same final project semester after semester. It sounds like she's too lazy to do that.

The reason this is an unwinnable fight for you is that when someone is this lazy about their job, it's because that's the kind of person they are, and they are strongly motivated to keep things the way they are. Offering sound ethical and educational arguments to the contrary will not work, because an unethical person will never agree with an argument when agreement would mean having to change their behavior.

If there are any issues, they are between the instructor and the students. One would assume that if a student approached the instructor with questions about the grade, then she would be responsive to that request. If she is not, the student has the responsibility to pursue recourse.

The best you can do is to grade as quickly as possible, so that students will have the ability to ask questions soon.