Adjectives decline as well as nouns, yes.
It is partitive here because it is used as a mass noun. You might as well say "tämä limonadi on makea" in which case you would be talking specifically about the glass in front of you, for example.
But neither do we use 'lemonade' to mean a generic soft drink. 'Orange lemonade' would be 'orangeade'. 'Lemonade' is something that tastes of lemon. 'Soft drink' is probably the best British English translation for 'limonadi'.
I also disagree that 'pop' is somehow un-British; when I was little (which was some time ago), characters in the Beano were always drinking 'pop'.
I also see that the first citation of this sense given by the OED is from 1812, from the letters of Robert Southey.