まさか ... なんて is a common framing pair: 'surely you don't / the very idea that you would X' — the follow-up と思わなかった ('I did not think') is understood and left unsaid. The sentence-final わ is the feminine emphatic ending. ストーキング is an English-derived loan used directly as a する-verb, typical for modern concepts imported whole. 油断する 'to be off one's guard / let one's attention slip' is a high-frequency idiomatic verb with no neat English one-word equivalent.