Hey John, I imagine your questions aren’t relevant for you now 8 months later, but in case they are… I’m very much still learning about all this as well so can’t answer all your questions, but my understanding is:
Yes you can switch back to the original Journal theme if you mess something up. You would just redownload Journal fresh and upload that to your site.
And yes, you would have to repeat this process with new updates to Journal. You could also just skip the update if your custom version of Journal is working, but if you want to both get the update and retain your customizations, you’ll have to merge the two somehow. This thread helpfully lays out the 3 basic options available to you.