As an independent author you can do what you wish. The only restriction is if you are in the Amazon KDP select program then you have promised Amazon exclusive use for a cut of the Kindle Select pie. I also distribute my books on all the other platforms, and for my free sci-fi book host it direct on my web site and on my Ko-Fi shop (the 'buy-me-a-coffee' site). Selling directly and collecting money requires a bit too much work but technically you could do it.

Technically, yes, but Amazon customers probably wouldn't benefit from that. I don't currently distribute or sell books directly because that creates a tax burden. So it's probably best to let the various stores handle it. I still want to sell books but I don't want my readers to be restricted by DRM for a book they paid for. The honor system is fine for me.

Edit: I now realize you might mean in the Amazon KDP UI. I don't see a way to upload your own.