Is the official CFAI online mock exam designed as one full test?
Meaning, is the morning session the typical morning essay style questions just in multiple choice format?
And is the afternoon session the typical afternoon multiple choice questions?
Or is it a mashup of different topics? Or is it designed as 2 separate afternoon mocks?
I’m saving this test for the end when I can unload on it. 
No, you should just look at it as just prep for the PM section of the exam.
Look at that as you may have done if you did the online topic tests. So plenty of practice for you between the mock and the online topic tests.
Keep in mind the most recent note that they may vary the number of questions in the PM, so don’t always expect 6 questions for each vignette.
If you want prep for the AM session, they have the old exams from 2016 to 2018 posted with guideline answers, they are located in the candidate resources --> prepare --> tips for taking the essay exam. I highly recommend you do all of them and then go and look for older ones online.