Yes, re-mapping the fields to align for the newly added column can be very tedious if you have to drag field names around to align them correctly.
If the first row of your spreadsheet contains column headers with exactly the same name as your fields in FileMaker, you can select "Don't import first row" and then select the matching names option from the arrange by drop down to automatically align all fields by name.
An import button definitely means that you have an import script. You can enter layout mode and double click the button to find out the name of the script performed by the button. You can then open Manage Scripts to find and update this script. If you open the script for editing you should be able to find the import scripts step where you can click the step to select it, then click the specify button at bottom right to open up the same field mapping dialog. You should be able to correct this misalignment of the field mapping there once and then see this order retained until the next time the excel file's layout is modified.