It’s hard to have so many issues in one thread. In any case, clearly your environment has something wrong with it. You still haven’t indicated how your Group Scans are set (which is different than your once a day scheduled scans). Those happen many times over the course of the day.

In any case, that wouldn’t interfere with manually adding a device, and generally scanning a single system should work. How is your DNS? Sometimes, when things cannot be found, it is generally an indication of a DNS issue. There are a lot of posts out there regarding the inability to scan single systems that we don’t see on the network at all. If you search through the Community, you’ll find a lot of posts around all the questions you have (aside from the potential manual problem for which this thread originated).

Here are some troubleshooting tips for that single device: