A. Yes, you certainly can.
Now, there is something special we do for Shabbos called “Lechem Mishneh” the two loaves of Shabbos, which are supposed to be whole and uncut before the blessing. But if that’s not possible due to the circumstances, you still can and certainly should say the HaMotzee blessing.
About sliced Challah for HaMotzee, there’s a Chassidic anecdote from Jerusalem that rings in my mind. Reb Alter Simchavitch was a fiery and dedicated Chassidic mentor (“Mashpiya”) in the Jerusalem Lubavitch Yeshiva called Toras Emes. Once a rabbi from a non-Chabad yeshiva came to his home for Shabbos lunch after the prayers, and he was surprised to see Reb Alter, known to be a pious and spiritual person, bring out sliced bread and herring for the meal. (Remember, this was a time of poverty and limited resources).
The visiting Rabbi asked, “Reb Alter, what about Lechem Mishna? Seudas Shabbos?”
Reb Alter answered, “For a Mashpiya in Lubavitch (for someone who inspires spirirtuality) this is also too much!”
The truth is that Chabad philosophy is about integrating and fusing the spiritual and physical, elevating the physical of our lives to a higher plane. Chabad is spirituality that’s down to earth and relevant and tasty in our world.
But even with that being the overall Chabad perspective, Reb Alter’s response is still valid, in that it reminds us not to get too carried away or fixated with the physical even when in the performance of a Mitzvah.