>the apparent absence of word-initial [b] and [p] in PIE,
You are right about /b/ but not /p/. There are 118 PIE roots with initial
/p/, and only 28 with /b/. Initial /p/ is less common than initial /k/,
but more common than initial /t/, /k'/, or /kw/. Surprisingly, initial
/g'/ is even less common than initial /b/.
Though precise numbers depend on whose list of roots one uses, the figures I
have for initial consonants are:
s 211
m 138
k 166
bh 128
p 118
t 94
l 90
k' 88
dh 66
d 58
gh 57
g 51
r 51
n 47
g'h 44
gw 36
b 28
kw 22
g' 20
gwh 12
Peter