I will answer your questions in the order they are posted.
Yes, this is the way to add a set session value server behavior.
You can use a dynamic trigger, set it to the posted service select list.
Yes, you can have use the same name as the posted element, and it's value for the value of your session var.
For the value use the lightning bolt icon to select the select list.
The form is on the confirm page, you would add in a hidden form element like the others, but the name will be the same as the name from the checkout form, and the value will be set to the session variable.
To set the value select the element and go to the bindings to select the session variable and bind it.
For question d/ I do not understand what you mean on this one.
