Chord progression, instrumentation and structure are not copyrightable. You can only copyright lyrics and melody. They are different, so we have no case here.
You should charge him for custom work.
Could be pastiche but not plagiarism. Probably the client said “we want something like Beautiful Day”.
Profile graphics are not the gist on AudioJungle since people come here for audio, not your graphic design. Just avoid bad fonts, colors and animations that can make you look an amateur. The rest is superfluous.
Yes, just make sure to give a proper credit to the original author in the description and provide a link to the license.
If the license includes a ND or SA clause, then no.
Having a space in the beginning of line causes this.
Yes, you’re right, it’s not the problem to filter the watermark out but from my experience when someone is determined to use a song without paying he will find a way no matter what you do. In most cases they will simply use the watermarked version, you can hear them even in TV commercial.
Watermark serves a symbolic purpose rather than practical, like a small fence in the front yard – it shows that it’s yours but can’t stop anyone.