If you ever read the warrior diet, by Ori Hofmekler, the answer is the half and half is ok. There are so many variables in that simple question, I will not go into all them right now. Very complex and at the end of the day; it really is different for everybody.
It also depends on your end goal or objective. If it is weight loss only, it has no effect because it all comes down to calorie deficits. If it is to control insulin, probably is ok. If it is to control glucose, your ok, if it is to increase autophagy; in my opinion nobody can definitely answer that. It is extremely hard to test on humans. Again the variables are extreme, including just when your body is actually in that state, how long it lasts and when it ends after you break your fast.
I want to believe that the longer fasts actually have the massive benefits that some are claiming. Not saying they don't. Just at this point the science is not conclusive and will not be for a while. This is a hard study to pull off, and why would somebody fund it. Who makes money from the results? No special foods, no pills, no special supplements, just electrolytes., do not need a book to get details, etc etc. Just not high on the funded research spectrum.
I would test both glucose and ketones, before coffee, and after coffee with cream. Then wait 2 days and do it again before coffee and then after coffee without cream and compare the two. However, again the variables are more complex then they appear on the surface. It depends on your daily calorie intake daily, the macro and micro nutrients you eat daily, sleep, cortisol levels, hormone levels that vary especially with women, so many things that have to be adjusted for and considered. But you can only try your best, and adjust for what you can. At the end of the day it is the consistency over time that will benefit your health the most.