Unfortunately, value lists in FileMaker that list values from a table are "hardwired" to the order in the field's index and this will be ascending order in every case. There are calculation fields that some folks have played around with that produce a series of spaces where the number of spaces dictates the order. This is then used as an invisible second column in the value list (invisible due to using spaces) and you specify that the values be sorted on this column. It's clunky and limited to fairly short value lists. It also limits you to a single visible column of values as the second gets used to control the order.
If you really want the list in descending order, you may need to use a portal of values where you click a row in the portal to select your value. The portal can then be sorted in descending order where the value list based on the same table cannot be.
Feel free to let me know if you want a detaile description of either method. They both have their drawbacks.
Don't think there is a "best practice" for your second question, just different approaches that each have their strengths and weaknesses. The "best practice" is then determined by the needs of the project and the preferences of the developer.
I'd be inclined to duplicate the records and then update each newly created duplicate with any changes specific to that record as this reduces the data you may need to move from the original to the "near duplicate", but that's only one of several approaches you can use here.