@smellsofbikes I don't mind translation question (to the extent I can answer)! I don't know the i/na adjective (I guess this is JP learner lingo) but methinks I got the gist. In JP we need ソフトな b/c ソフト itself is almost like noun ("softness"). But if there was no しっとり in the beginning, i.e., if it was ソフトカスタード風味, then you don't need な. I guess it's because しっとり modifies ソフト...if you have しっとりしたソフトカスタード風味 then it's ok without な after soft.
EGGGGG..does this explain???????????????