In order for your AMCAS application to be sent to schools, it needs to be verified.
In order for it to be verified it must be fully filled out AND your transcripts must all be in.
If your transcripts are not in, it doesn't matter how early you submit, you won't be verified and your app won't be sent to schools.
So, the MCAT is not really the issue; as long as your score comes in by late June/early July, you will be in the first group of applicants considered. As soon as your score comes in it is automatically attached to you AMCAS application; you don't have to do anything else.
However, if you aren't verified....none of this matters. So, when the application opens in May, you should get your AMCAS number and immediately have your transcripts sent to AMCAS. That way they will be there well before you submit and then you can be verified ASAP.
I am super tired....so hopefully some or all of that made sense🙂