Are all mortal sins of equal malice? If not, then are there degrees of "infinitude"?
Of course not all mortal sins are of equal malice.
Yes there are different degrees of infinite offense.
Venial sins are also infinite offenses against God.
All sins are infinite offenses because God is infinite and perfect, and in committing a sin we corrupt Creation. The act of sin can never be undone, it can only be atoned for - by Christ's suffering on the Cross.