Pg. 36
The あたし on the 山田 is one of those cases where the furigana is used to create a double-meaning.
あたし is what she says outloud, but 山田 is used because (spoilers if you haven’t caught on yet) she is meaning her usual customer-facing 山田 persona. The implication then is that she will try again as 田山, which is what she does next.
You’ll see furigana used this way from time-to-time, as a way to introduce double-meanings.
My usual spiel in the ABBC threads (drop down arrow just above Grammar and Resources subheading) includes a section on this usage of furigana, if you want to do a bit more reading on it.
This is called 義訓 (ぎくん)