Etymonline indicates that spelling with lef- dates to the 14th century, but that the origins of that spelling (and presumably its associated pronunciation) are “mysterious”. The word comes originally from Old French, and according to the OED, Old French replaced word- and syllable-final [w] with [f]; for the Modern French word lieu, this is shown by an Old French spelling variant luef. Both forms, whyever they exist, just happened to stick.
Forgetting about the "doubling" of the consonant for a second, in English, some consonants, most notably c and g, but also s, t, x, and the pairs ch and th are softened when followed by softening vowels (usually i and e are the softening vowels, but a and io soften t, s, and a softens x for example).
Most of the "softenings" originated in Late Latin as a result of either intervocalic voicing or palatalization before front vowels.
Now what does it have to do with a double c?
Let's look at an example: eccentric
The first c is followed by a consonant, c, so it is pronounced as a k, while the second c is followed by an e, so it is pronounced as an s.
Putting those together gives a ks sound.
On the other hand, in a word like occasion, the second c is followed by an a, which doesn't soften the c. So both cs are pronounced as k's, which is in turn pronounced as a single k sound.
Best Answer
Answer to (2):
We memorize each exceptional word. In the grand scheme of things, this is actually not a huge number of words. Consider Chinese and Japanese, who have a large number meaning-based characters that provide little to no information about their pronunciation at all. If memorizing the pronunciation of thousands of characters is possible, then memorizing English words with exceptional spelling seems almost trivial.