A comparison only works if the two halves match. Same size, same shape, ideally the same angle.
If one photo is larger, closer or brighter than the other, the comparison stops being about the change and starts being about the photography. Matching size and shape first is what makes the difference believable.
Shoot the after from the same spot as the before whenever you can. It is the one thing that cannot be fixed afterwards, and it is what separates a convincing comparison from one that invites doubt.
Label them. Before on the left and after on the right is the convention in most places, but conventions are not universal and two small words remove all ambiguity.
Side by side for tall photos, stacked for wide ones. Match the layout to the shape.
Add them with the text tool after combining.
No.