I thing you do not understand the idea of conditional formatting...
I am not sure what exactly is not clear in the explanation.
1. You CAN ONLY format (which includes adding css class, making the field clickable or not, adding a condition) the calendar fields that have dates from the database!
2. You CANNOT format the rest of the calendar fields.
There is no logical reason to block any FREE dates (not reserved/stored in the database) from being selectable... Have you ever used any reservation system?!www.dmxzone.com/demo/php/DMxzoneCalendar2/WiterResort/index.php
check this showcase.
You cannot click on the dates that are RESERVED ( stored in the database) as someone else selected them! You can only select FREE dates... that's how the datepickers have been always been working.
The topics you mention explain exactly the same....
As for the condition - you can apply different conditions to different RESERVED dates. Example:
You have 3 people working for you, and each of them can use 1 day off in December, so if John selects 12.12.2015 it gets one class(blue color), if Peter reserves 14.12.2015 it gets another class (red color) etc...
That's a datepicker it's idea is to select dates and not be able to selects dates that are already reserved by someone else!
PS - i think you are confusing a Datepicker with Event Calendar.