I have never had a problem with MKL on intel machines. So I am not sure what might be the problem.
YOu don’t need to install Anaconda because ISiS comes with all dependencies included. Whatever you install on your system should not be used anyway.
If I am not mistaken I had High Sierra installed on the machine I compiled ISiS with. So there should not be a problem on that side either. in any case Anaconda supports MacOS versions
The MKL lib has been added in version 1.2 8, so if you install 1.2.7 there is no MKL, and if really MKL is the problem for you, then version 1.2.7 should at least run. Can you try that?