the reccomendation against pre-mixing was in regard to cream and aloe vera gel which would be very difficult to get an even dispersion- with respect to those it is far better not to pre-mix. However the solution can be used either way- pre mixing is nice from an ease of application and measuring standpoint.
yes shake bottle vigourously, 10-15 sec, each time before application.
If you let it sit the oil will slowly float to the top- the yohimbine will stay relatively suspended in the remainder of the ingredients- ie. it will NOT settle on bottom.