Sure, and that hasn’t changed. The only difference in Leap 42.3 is that you have to uninstall drm-kmp-default, such a thing didn’t exist in previous openSUSE versions.
But you still need to reinstall it after every kernel update (was also the case on 13.1), unless you use dkms to automate that.
Mesa updates will overwrite your nvidia installation as well (the OpenGL libs, should be “fixed” in Leap 15 due to the usage of libglvnd), and on 13.1 even xorg-x11-server updates did, that has been mitigated in later openSUSE versions (by using update-alternatives for libglx).
And I suppose you mean “blacklisting” nouveau here, because you cannot remove the nouveau kernel module (it’s part of the kernel package), and you don’t have to remove the Xorg driver anyway.
But not even blacklisting should be necessary anymore I think, AFAIK the .run installer does that for you meanwhile.
The problem with Leap 42.3 and this method is that you can´t do i offline. It seems as if you need to be online. The packet is not in the cache. Or i didn´t find it.
If you mean the rpms from the nvidia repo here, then yes of course you have to be online.
You can download and install the packages manually though (with “zypper in xxx.rpm yyy.rpm …” or “rpm -i xxx.rpm yyy.rpm …”), and add the repo later to get updates.