The best way to get a specific sort order on a Filter Pane is to use a dual data type. These have both a numeric and a string representation. This means you can have a different sort order to the alphabetic one.
So, to have a filter that sorts the words one to four correctly it would be:
Dual(Number, ApplyMap('Map_Number', Number, 99)) as Number
You will see that a filter pane based on this will sort correctly.
There is no "only one selected" or triggers in Qlik Sense (as there was in QlikView) so an elegant way to solve the default issue is to use Set Analysis in expressions.
First you would need to create a variable that returns the first sorted value (this can then be used in legends also). In this example the name would be vFirstValue and the code:
The code is a little convoluted, but basically you are finding the lowest numeric value and then find the string equivalent that goes with it.
You can then use this expression in Set Analysis expressions, so regardless of what is selected only the first sorted value is used in the expression:
Hope this all makes some kind of sense?
I've attached a QVF in the hope this makes things a bit clearer.