Short answer is NO one cannot commit a mortal sin without knowing it.
The underlying basis for a sin being Mortal is simple. A sin is mortal because it is a willful rejection of God, of God’s Love, of obedience to God.
If we Love God as we should then we should not wish to offend Him in even small matters, let alone large ones.
If I offend God, even in a small thing, I consider it (the sin) a grave matter, whether the object of the sin itself was grave or not. Why? Because I do not wish to offend someone I Love. I do not wish to alienate myself from them either in large matters or in small ones.
The reason I write the above is that when we start to look at the “legalistic” definition of Mortal sin we tend to start trying to differentiate between the varous “stages” of consent, the various levels of “gravity” and the various forms of “knowledge” that can go into determining if something is a mortal sin or not, “under the Law” as defined by the teaching of The Church. Such “defining”, without the underlying rock of understanding, provides the evil one an opportunity to mislead and to gradually draw us away from God’s Love, first in little ways and then in larger ones.
Looking at sin in the more unifed sense of disloyalty to the one you love instead of the “legalistic” view provides a more firm basis upon which to avoid sin and build a strong and loving relationship with God. Remember that Christ Himself says that All of the Law and All of the Prophets, (and I dare to add that all of Church Teaching), are based on the Two great commandments of Love (Mt 22:36-40). Weigh all of your thoughts words and deeds, all of your understanding of Church teaching, against this and you cannot go wrong.
Peace
James