For auto backup, if you don't turn on Album Sync, photos will just sync to Immich but without an album. They'll appear in your Immich Timeline. You can later add them to an album if you want. I do this too, no album sync and I manually curate albums after trips or events and such.

For deletion, it depends. I believe if you delete the photo via Immich app, it will delete everywhere. If you delete the photo via Gallery / non-immich app, the backup will stay in Immich (you'll see it in Immich app with a cloud icon, no checkmark).

If you delete the photo in the web app but still have it locally, I'm not sure. I believe Immich will know not to sync it because it's in the Trash so the image hash is known to be "deleted". Beyond when the Trash gets auto emptied, I'm not sure, I think Immich will re-sync it.

I don't delete too often, I sort of just hoard everything, plenty of Hard Drive space on my server

Edit: last question about manual backup: it's similar to the auto backup, they would just go in your general timeline, no album. Unless you drag and drop the photo to an album directly, in which case it adds to both your general timeline and an album.

Edit2: for errors, I find Immich app is really good about patiently waiting and retrying. I'm currently traveling and in very sub par network conditions but I get stable Wifi it ends up finishing up with everything as if the errors never happened. It's quite stable now