Yes the Speidel bracelet will continue to eat into the lugs as it is spring loaded at the lugs. Personally I'd put that on a leather strap. One way to mask some of the inner lug damage would be to fit a slightly oversized strap, ie a 19mm on 18mm lugs for instance as it would bunch up a little and fill/cover the damage.
In pretty much 100% of examples, a gold coloured watch with a steel back is gold plated, capped or filled, not solid. It is a requirement of hallmarking that solid pieces need to be fully solid. Of course it could have been modified with a steel back for inscription purposes but I doubt it. You do get mixed metal cases using solid gold and steel but not like this. More info will be found on the inner caseback inscription. It will be worth you removing the back with a non marking case removal ball to see what is written there.
I just realised you were asking about plated vs filled, I wrongly assumed you meant plated vs solid. It could be filled but it makes no real difference. Gold capping or filling is just a thicker layer vs plate and all 3 are treated similarly by the market with steel or solid gold usually being preferable. My money is still on plated for this. It's deffo not capped as the lug backs are gold coloured and the steel caseback suggests plate.
