The change from mecha sonic to metal sonic was a sudden change by the staff who were afraid that the idea would overlap with 2. While mecha sonic is a simple villain with no depth, metal sonic has become a wonderful character with its name and vivid descriptions.
I AM SO EXCITED FOR THIS ONE and a little side note: most people doesn't really like Sonic CD but as for me, it's something beyond than just a simple sonic game, time travelling mechanics, the color palette, the music are just incredible! Of course there are many different opinions on it but this is truly one of the most unique sonic games ever made
in a youtube video it is confirmed that the "creepy sonic" thing was supposed to look like mario BUT as sonic, i wont really believe it due to copyright but it sounds true.