This is a TCP error, the PLC refused TCP connections. The app has not even been able to establish a TCP connection. This usually means a misconfiguration somewhere, such as a wrong port number or an outdated firmware on the PLC, but can be other things.
The Siemens Logo is not officially supported -we did not test it- so I am not sure if it would work. The default slot number and rack number for SM are 0 (zero). This can be changed with the 'controller_slot' attribute. For a S7-1200 this is zero (the default) so it works with the default setting, but I do not know about the Siemens Logo. I will try to investigate this. Thanks for your feedback.
You can use 'internal' tags, but they of corse do not update according to any external source. Then you can replace them by actual PLC tags.
Back on topic, our early investigations point out that the Siemens Logo does not use the Siemens ISO/TCP communications protocol, or at least it does not do so in the same way than S7 controllers, so it does not work with ScadaMobile. Unfortunately, Siemens is very closed with regard to give any details on communication protocols, so we may need some time to figure out what to do. It already took a long time for us to support S7 controllers for the same reason. So I'd suggest that you replace the Logo by a S7-1200 for now if this is still acceptable for you, as it is fully supported, and it works flawlessly. I'm sorry for the inconvenience.
No, you can do the opposite. If you use an actual PLC tag then SM will attempt to connect to a PLC using the default settings, you can't prevent this. I thought you wanted to create your project using internal tags and latter on replace them by actual PLC tags. This is possible, but not running the app with no PLC unless all the tags are internal.