Before and after only works when both pictures are in the same frame. Sent separately, nobody lines them up mentally.
Side by side suits a comparison, because the eye can flick between them. Stacked suits a sequence that someone will scroll through, and works better on a phone where width is scarce.
Images of different sizes are scaled to match on the joining edge, so nothing is stretched and nothing is enlarged past its real detail.
A small gap in a contrasting colour makes it obvious these are two pictures rather than one odd one.
Yes, add as many as you like. For a grid rather than a line, use the collage tool.
They are scaled to match on the joining edge, never stretched.
No.