We usually do conferences in cryptography/security, and most of them have page limits: CCS, USENIX, NDSS, S&P, CRYPTO, EUROCRYPT all have page limits (some allow appendices, which reviewers are not obligated to read).
Yes, if you count the emissions for the stuff manufactured in China for (not only) european customers.
If you look at consumption-based emissions[0], China is at 7 t per capita, which is 8 t less than the US, 2 t less than Germany and about par with Denmark, Slovakia, and England.
There is no known polynomial time algorithm for TSP. AFAIK there are quantum algorithms for TSP with quadratic speedup in comparison to the brute force method (still exponential though).
The one thing quantum computers are (currently and theoretically) able to solve efficiently is the factorization problem, but are very much limited by low qubit number, i.e. engineering problems.
That's an interesting question! Disclaimer: In my current research, I'm focusing on steganography on ML-based lossless channels. Not an export on image-based steganography.
As far as I know, image based steganography usually uses random noise added to a cover image and a preshared key to decide which noise is actually encoded information. Since printers and scanners are a lossy channel, the noise will deteriorate. So I think this is a hard problem -- this might be possible with some kind of error correcting code as used in QR codes and alike (or maybe even use QR codes as the cover channel)
The paper this project is based on is the starting point for my thesis. I have found little discussion about this project (and have not found a posting to HN about it), even though I think it shows a very interesting approach to text-based steganography.