This is my suggestion. It sounds like you are already using unique names for your magnets, which is a good start. If you rename the magnets in Poser, it will only change the display name, not the internal name, so you need to do the renaming in a text editor, or a cr2 editor, so that you can change the internal name as well. You need to do the renaming by 'Search & Replace', so that you catch every instance of the name in the file, and you need to rename the Base, Mag, and Zone.

As you have found, Poser will sometimes add a "_1" to the name. The way round this is to put the "_1" in the name to start with. If the name already contains the "_1", Poser can't change the name by adding it, because it's already there. The combination of unique name and the _1 should insure that the magnet always loads with the same name.

Of course if you were to add a second instance of the same magnet, the name would still change and the second instance of "MagLt_1" would become "MagLt_2".