Sending six screenshots means the reader has to piece them together. One tall image tells the story in the order you intended.
Screenshots at different widths stack into a ragged column with gaps down the sides. Cropping them all to the same width first is what turns a pile of pictures into something that reads as one continuous image.
Crop the repeated chrome out of everything except the first. Six copies of the same status bar and address bar running down your image is visual noise, and removing them makes the content flow.
Check the edges before sending, harder than usual. A stack of six screenshots is six chances to leak a notification, a tab title or an unrelated message.
Yes, the same tool stacks horizontally as well as vertically.
A tall stack can be. Compress it afterwards if you need to send it by email.
No.