I ran MatterControl as 'Root' and still have problems. When I re-scale an object, then hit save as, it changes the icon to a question mark. If open the file folder, I see an '.AMF. file, but attempting to view gives me a blank preview screen.
For the Flashforge issue, could you post your slice settings and your old/new X3G files for comparison? In fact, if you could export a gcode file and post that too it would probably be helpful.
As for the Robo, could you tell me a little more about how it freaks out? It sounds like maybe some issue with your Z axis stepper motors. Does it move up and down all right when you use the manual controls?
Yes. Load your first model into MatterControl, then go into Edit mode. You can then drag and drop additional STLs into the edit view, and they will be added to the same print. You can also hit copy to print more than one of the same object.
It seems like you've got things backwards. PLA is supposed to be stored in a dry environment. Moisture is what causes it to have issues. This effect is called hygroscopy, and there's more information about it in this thread.
In any case it sounds like you just had a bad spot in the spool. If you find that the rest of the spools is excessively brittle, contact firstname.lastname@example.org and we will replace it.
i find with certain poor quality filament the moisture content is very high, this will steam and pop a lot, but if you do want to use it,i find if i slow down the print speed to 20% and heat up to 240 and walk away... tbh rather get some pla.