OK, so deleted many more photos using the Gallery and Photos/On Device and I can confirm that they've remained in Google Plus. I'll switch from primarily using the Gallery to the Photos app.

One thing to note if you're doing this: You have two choices when doing auto-backup of photos to Google+ actual size/resolution (whatever you have as your default) and web sized. (Google may use different terminology.) You can backup an unlimited amount of web-sized photos, but if you backup at the original resolution, it counts against your Google+/Google Drive total capacity. So if you want the unlimited option but care about your photo resolution size, back them up to another storage option, too.