How can I protect my (engineered) hardwood floor from scratching?
I really doubt you can sand your floors.  Some engineered floors can be but this is a very small percent.  And this would be a diy because pros won't want to be responsible for the thin top layer sanding through.

One of the things I have found with engineered hardwood is the clear coat varies drastically from different types.  I have tested a lot and some scratch by pushing a penny across it and others I have had to stab full force with a screwdriver.  I am guessing yours scratches with a penny.  

So the only cost effective thing that would definitely help would be to add a couple layers of poly.  You may need several layers.

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