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Looks like regular tarnish to me too -- and I'll be curious to see your results with paste wax because I may try something similar. I do NOT remember the Star tarnishing in the time I had it, but since I got it back from the environment that turned it black, it seems to be tarnishing all on its own, and I do not have acidic sweat and am certainly not resting any body part on the bell. Maybe I just didn't notice it before, but I am noticing it now, and would do something like put Turtle Wax on it if I thought it would preserve the finish and "do no harm."
I would never buy a new silver-plated instrument because of this problem, but both the euph and the Star were bought used and I took what I got, just like you did.

Is there any reason why silver plated instruments are not also lacquered?

I have also wondered about some sort of clear coating over the silver plate. I'm guessing that it will still tarnish...?

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Best I can make out, there isn't anything that will make a given silver plating more or less apt to tarnish, short of some kind of sealant. I mean, some metals form their own protective oxide layers, but not silver.

While a sealant will work - anything that doesn't itself have some reactive sulfur in it - it has the same liability as lacquer on brass. When it's compromised, you get a wretched splotchy effect.

I've had great results in the past with TarniShield, which both polishes and leaves a protective anti-tarnish coating on the silver. Throw some anti-tarnish strips in the case or gig bag and you should be good to go.