I would bring it into https://rasterbator.net and choose the highest detail setting (many small dots) and make a poster of it that way and then print it myself on however many A4 sheets I wanted. Glue the paper sheets to cardboard and hang out up as a collage that looks great from a couple feet away. You can make anything this way. Do a Google search of "rasterbator" and see for yourself.
I don't think so? It's not really what snapseed is for imo. I use another free app for things that involve detailed stuff like that.
This is the app I use: https://play.google.com/store/apps/details?id=com.iudesk.android.photo.editor
Try Google Lens instead of Snapseed. Take a screenshot of the QR code, open it in Google Photos and tap the Lens icon. I've also read you can share your screenshot to Google Lens to do it.
[EDIT]: If you've got the image saved on your phone already, you can skip the screenshot. Just open the image with Lens (through Photos, sharing to Lens, etc.).
I think the app is broken (for some devices) since the last update. There are a lot of people commenting about not being able to save pictures in the Play store: https://play.google.com/store/apps/details?id=com.niksoftware.snapseed&hl=en_US&showAllReviews=true