Hello!
I recently encountered a similar issue similar to your first. The game I am currently working on I wanted to have specifically blend into the webpage background. You can see the end result here.
As for how I completed this, I found the incredibly useful (Unity) Better Minimal WebGL Template by Sean. Following the steps listed on this page to download & install the package into your Unity build will prove very accessible. The only thing I will state is aware that you also must be aware of your itch.io Embed Options when uploading your build to itch.io itself. You should no longer have to account for any border as you have done, but rather just input the actual desired game size.
I currently don't have any insight for your second issue, but hope that you are able to figure it out soon.
I hope that this was helpful!