The last sentence is not grammatical. Also, only a person can be "more stunning than ever" because a person can be seen in different outfits or looking different on different occasions.
If you say the painting is stunning, that's OK but it can't be more than ever because then it would sound like a person or that it had a make-over.
Here's a transformation to see how this pattern works:
1) He returned and closed the front door, making sure it was unlocked.
1.a) He returned, closed the front door and made sure it was unlocked.
2) We played a fun game, trying to remember the day’s coaching tips.
2.a) We played a fun game and tried to remember the day's coaching tips. [as we did so].
3) The bullet missed him, passing over his head.
3.a) The bullet missed him and passed over this head.
With this one: The finished painting, stunning than ever
you can't do that. First, it is not a sentence with a subject, verb and object.
"The finished painting and stunned more than ever." is a no-go.
And this: The painting was finished, stunning more than ever. is poor semantically as explained above.
The painting was finished and was more stunning than ever.
compare that to:
The bride was dressed, more stunning than ever.=
The bride was dressed and was more stunning than ever.
The last one works.