i have a question, im a bit confused- why is alif not considered a leen letter despite the fact that it is preceded by a fatha and is sakin?
Answer
The alif is a medd letter, and it is preceded by a fathah as it root structure. There is no alif if not preceded by the fathah. The alif is lengthened two vowel counts when not followed by a hamzah or a sukoon.
The leen letters, which are the
saakinah preceded by a fathah, and the 
saakinah preceded by a fathah are not medd letters. Instead if there is no sukoon (temporary or otherwise) after the two leen letters, there is a lengthening of less than two counts. It is longer than a harakah but short than a medd letters. Without the lengthening of two vowel counts for the alif, it would not be an alif, so it cannot be a leen letter, since it is inherently lengthened two vowel counts.
Insha' Allah this clears things up for you.