I have struggled to get great results from the B&W Mixer as some color slider movements has a tendency to cause nasty edge halos around high contrast edges such as leaves and sky or mountains and sky. Moving the blue slider for instance only 10 points can lead to significant edge haloing. Is there a method to avoid these issues?
No unfortunately not. You will typically see these on blue skies when you really try to darken the sky by sliding the blue slider way down. It is caused by the way the color range masks are calculated and is very hard to avoid completely as there is no feathering setting to the color range masks and you will almost always see these with extreme settings of the mixer. When I get this problem but still want darker skies, I usually switch to Photoshop and use a channel mixer adjustment layer set to black and white output since that does not have this issue as it doesn't use masks. It is less versatile to control the output but usually does just fine with a little playing.