Short answers, pulled from the story.
Large eyes became a permanent fixture in manga since the early 20th century. Illustrators Yumeji Takehisa and Jun'ichi Nakahara published works featuring female characters with large eyes during this period.
Spirals inside eyes indicate dizziness or overwhelming confusion while flames convey anger or vengeful emotions. A single large X represents crying rigorously or death in comedic contexts and love-hearts indicate infatuation.
The symbol for prominent beads of sweat on the brow was added to Unicode 6.0 as an emoji in 2010 with code U+4F4A2. This visual element represents embarrassment, exasperation, confusion, dismay, and shock beyond normal sweating conditions.
Transparent adhesive sheets manufactured with distinctive patterns introduce shading without hand-drawing time-consuming details. These sheets often feature dots or hatching but sometimes include flashy effects like stars or explosions.
Manga artists play on childlike neotenous facial features to increase protagonist appeal according to psychological research. Youthfulness appears as physical traits like younger age or pigtails alongside emotional traits like naive outlooks.