I am glad you are doing your homework and you have support and help here.
Yorkies can be so individual, you can not expect for the same general signs...
The fact that she wants to go under the bed is a "nesting" sign for good.
She may not be very happy with her whelping box or , which is more likely, she has no clue what the whelping box is for since it is her first time !!!
you are on the good start. If i can be of any help you can privately message and we can exchange phone numbers....
the temperature going back and forth is also normal and also a sign that the labor is very near. her temperature will drop to 98 eventually ... then it is really happening !!!
Good Luck !!!! Keep us posted !!!