I've been using the XPS 9560 full-time since February, and have experienced precisely 0 of the issues you describe whilst running a number of Linux variants, including several Ubuntu variants, Fedora and Solus.

> So I don't even have a way of getting a working version of Ubuntu on this laptop if anything happens to my hard drive.

You could simply download Ubuntu directly, or any other distribution. Dell isn't doing anything special to the version they distribute.

> For example, updating BIOS to a version that fixes an important CPU bug causes the computer to freeze and shutdown every few minutes at random.

Have you tried talking to Dell about this issue? Their support, particularity around BIOS issues on their XPS and Precision range has been fantastic in my experience, and they often supply pre-release BIOS versions if they believe it will help, otherwise they tend to replace the laptop.