My problem is, that my Vodafone Smart Prime 6 (VF-895N) with Android 5.0.2 mounts external sd-cards as /storage/sdcard1. On other devices with "settings" - "storage" there is a menu to change the default write disk. This menu is missing on this device. Is there any possibility to mount my external sd-card as /storage/sdcard0 and the internal storage as /storage/sdcard1 without rooting?
The internal storage is present as 'storage/emulated/0'. There is no way you can "load" the internal storage as 'storage/sdcard0' or anything else. Internal storage is always present as 'storage/emulated/0', you cannot change it.
Even with rooting I think you cannot reload the internal storage as sd card. Default storage location cannot be set like that. If your phone doesn't have that option, it's a pity and I believe there are apps which allow you to do so (caution: I haven't checked any).