I just checked and it is not possible to add a serilaised item to the same location twice to end up with two of the item with the same serial number!!
I think this was just because you had the item set as controlled - BUT not also serialised. It must be set to serialised if you wish to only have one of it.
So in answer to Question 1 - yes you can add multiple items of the same LOT/Batch/Roll when an items is controlled (but is not seriliased) - and NO you cannot add multiple items of the same serial number when the items is defined as controlled AND serialised.
In answer to Question 2 - if you select the location where the stock is and you hit the remove items in the stock adjustment screen the serial items (or LOT/BATCH/ROLL references) that currently exist at that location display for you to select - as we agreed was a good idea - and it IS implemented.
It is my guess that you have the location MEL selected and you really wanted "Portacabin1"