Unfortunately, we don't have experience with DevExpress reports.
You need to give a proper permissions to user. It should be enough. This error means that user you use does not have the appropriate permissions to this operation.
Did you check the system requirements and your server meet this requirements?
You can check it here: http://docs.grandnode.com/system-requirements
Out of the box it's impossible. In the GrandNode 4.40 version we are planning to implement new feature related with prices, but it won't automatically detect customer currency. It can be assigned to store. If you want to create it in current version, it need to be a customization.
You need to create a three different themes, then in General Settings select multi store configuration and select different themes for each store.
Yes. If you configure multi store correctly, it will work as you described. Remember about "Ignore multi store rules" setting in Catalog settings. It should be unchecked.
Can you show me appsettings.json file?
Thank you for suggestion. We will consider it.
No, please try to add this setting in the web.config file: