To answer all parts of Tes's posting:
1. All she was doing was nothing but -ing
Google shows no examples of either all she was doing was nothing but..., all he was doing was nothing but..., or all they were doing was nothing but....
We did nothing but running for gym in high school
is OK because "running" is the name of the school activity. It's akin to saying
We did nothing but drawing in our art classes; painting wasn't included
This is different from saying
He does nothing but draw fantasy figures all day
You may well have heard such a construction as 2c)
(2c) She did nothing but crying the whole afternoon.
...in popular music. Song lyrics, which are a form of poetry, are not always a reliable guide to English usage. Writers of lyrics are always striving for artistic effect. That's what makes them fresh and appealing.