i'm assuming you realize that vowel system is very unrealistic, but, that vowel system is very unrealistic. maybe e > ə pushed o downwards, but that'd still be weird, since it's already got a rounding contrast in nonfront vowels. e > ə would make some sense given the presence of /je/... hm.
the vowel system is back-heavy on purpose, yes, to give it lots of ways to fall over in the descendants.
Manchu seems to have something similar, http://en.wikipedia.org/wiki/Manchu_language#Vowels but the back vowels are even closer together and there doesn't seem to be a rounding distinction. i like the idea of /u/ being fronted initially. maybe word-finally, though.
For Proto Martial I was moving towards a vowel length distinction with something like
ə > e
iə > e:
a > a
ai > a:
ɒ > o or o:, depending on environment (after ß, which i wrote as an eszett because it makes long vowels)
ɯ > u
u > u:
i > i
ui > i:
idk about allophonics except the ß thing i mentioned
proto-RM also ends up with vowel lengths maybe except just a a: i i: u u:
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