the direct answer to both questions is yes.
i use corel photo paint to make and adjust shading. when doing just one face , most of the time, splitting and photo shopping is not needed. when there is more than one subject, they are almost never in the same light, or if you are using a different photo for each person to make a portrait that does not exist in real life because they never posed together.
thus my parents. each face was a different size in their respective picture so i made my dad's head bigger. i was not pleased with how my mom's neck turned out and it was originally stacked 8 deep but i went back and changed every one. i wish i kept a picture of the repair.
in a single picture i may split the people and rejoin. my daughters were in the same picture but one face would end up darker. so i cut them apart made a pattern for each and then put them back together in a way i wanted.
people recognize others mostly by the eyes and/or face shape. that is why mostly you can tell who someone is when they have dark glasses on or see just there eyes when they are wearing a ski mask. that is why no matter what i do i never change the eyes or the shape of the face.
i try very hard to control the photos and only let them be the guide.
for me i take the time to create the portrait i see in my head, it does not always match how the original picture looks.