What is the ruling on attending a lecture of another group, or another section of the same group, given that the time of some lectures is not suitable for the student, and this may lead to a lack of available seats for the concerned students?

The reference point for the issue of transferring between specializations is the university's regulations. If they permit it, then there is no harm. If they prohibit it, then it is not permissible. The excuses mentioned do not justify violating the university's regulations.