For one thing, it's 難しそう, not 難しいそう. This is important. The latter is ungrammatical.*
(*At least for this grammatical pattern. There is a separate ~そう for hearsay that comes after the full predicate, e.g. 鈴木先生はなかなか厳しいそうです。"I hear that Prof. Suzuki is pretty strict." Note that the hearsay ~そう is not used adjectivally.)
And the "seems/appears" そう does, indeed, function grammatically as a な adjective.
優しそうな目をしている
When な adjectives are used adverbially, they take に. Same for ~そう.
丁寧に案内してくれた
楽しそうに遊んでいる
etc. etc.