Plane is quite new, and reasonably comfortable, even in ordinary economy. No free onboard catering, however. For that you'd have to go Economy Extra, which on SK is a separate class. No automatic upgrade for anybody, no upgrade instruments available for anybody. You might be lucky with an op-up, to Y+ or C, but don't count on it. In my experience ticketed *A carrier doesn't matter when it comes to SK-metal flights. I have been upgraded and not been upgraded as a EB Silver/Gold both when flying on a UA codeshare SK flight ticketed by UA, and a SK flight number ticketed by SK. You're unlucky being ticketed in W as that is one of the notorious "25% earning" booking classes on SK. V is usually not that much more expensive, but will give you 100 % earning.

Not much point of asking for an upgrade intra-Europe; if you get one, they'll have a new boarding pass waiting for you at the gate. I don't know the UA arrival lounge at FRA, but comparing my experiece with UA operated lounges at LHR and in the US with the LH operated lounges anywhere, I would strongly recommend you to spend your time in a LH lounge. There are great showers (might be some waiting time in the morning) and usually proper food and snacks (as well as free booze). Eat well there, and the missing catering onboard SK won't matter. BTW LH metal flights to Scandinavia still has catering onboard. Sandwiches, soft drinks, coffee, beer and wine.