Wingfoot, re. question 1, without getting into the math of the situation, I would just say "yes". If I had to deal with this, I would prefer to replace all four brake assemblies, complete with new magnets, brake linings, springs, etc. I have spent far too much time and money attempting to replace parts of these brake systems. The whole backplates are surprisingly inexpensive, especially when compared to buying individual parts. If I had to economise, or just needed to get the trailer home, I would remove the magnet, and rub it on a piece of coarse emery paper resting on a thick flat piece of glass or steel until the ridge had gone. Then I would use some finer paper to smooth it off. There is obviously a limit to how much can be removed. The securing bolt heads should not be touched. That should be sufficient to pull the trailer safely to get you home. If you pull the trailer with the distorted magnet, you could ruin your new drum. Re. question 2, if you disconnect one wire nut behind the wheel and insert the ammeter between the two ends, you can compare the amperage with the factory spec. for the magnet when the brakes are applied. Good luck. Nick.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."