No, אלא doesn't mean "rather". If you wrote "but rather" it probably would have been accepted, but "rather" on it's own isn't a good translation.
I wrote 'but' and it was not accepted. The answer said 'but rather' ... which one wouldn't say in English.
What would "in three days", "..four days".. or in a 100 days be ? Is it always written in a one-word form like מחרתיים ?