What's the logic behind Lojban's sound system?

["uaxuctum" <uaxuctum@yahoo.es>]

Hello,

Lojban is presented as a "logical" language, but so
far I haven't been able to find the logic and consistency
of its sound system. Could anybody please explain me the
reasons for the choice of phonemes and permitted sound
sequencies? To be more specific, I'd like to know:

1) Diphthongs ai, au, ei, oi, ia, ie, ii, io, iu, ua,
ue, ui, uo, uu are all allowed but not eu, ou. Why?

2) As full vowels i and u are spelled the same as
their semivowel counterparts, how are diphthongs ii and
uu to be pronounced, as 'yee' and 'woo' and Arabic 'yi'
and 'wu', or as Arabic 'iy' and 'uw'?

3) Why is voicing used for all stops and fricatives
except x?

4) Why are there so many gaps in the phoneme chart?
--the labial order has two stops (p/b), two fricatives
(f/v) and a nasal (m), the dentalveolar order has also
two stops (t/d), two fricatives (s/z) and a nasal (n),
but then the alveolopalatal order has only two
fricatives (c/j) and the velar order only two stops (k/g)
and one fricative (x)--

5) Why does lojban use such a rare phonemical
opposition as /h/ vs. /x/, which moreover is found
in just one of the languages it takes as 'models'
(only Arabic contrasts them)?

6) Why, on the other hand, doesn't it use /N/, which
is a very frequent phoneme of Chinese and appears also
in English and Hindi?

7) Why are clusters of the kind 'st', 'ck', 'jm'...
allowed while others such as 'll', 'kn', 'nd' are not?

8) Why are liquids and nasals allowed as vowels, when
of the six 'model' languages only English uses them
that way?

Thanks in advance,
Javier