Some letters aren't letters

[Torsten]

Torsten: it seems that you are inferring from the lack of -a or -o words but the abundance of -y words that -y is an orthographic variant where those same words should end with -a and -o.

I'd accept that word-initial y- could be a variant of word-initial o- , but I don't think you're going to convince anyone here that word-initial y- would replace word-initial a- in all but the very rarest of circumstances. So this is already somewhat asymmetric.

Indeed, my statement was not specific enough. What I mean is that it is possible to replace each instance of [y] with [o] or [a]. What I mean is that [y] is a variant of [o] and [a]. (clarified)

The usage of [y] is very strict. There are 4184 word types with [y]. There are  3096 word types ending in [y] (74.6 %)  and 625 word types starting with [y] (15 %). Some of the remaining words are still words like [dydydy] or [chedydy]. Therefore words like [kydain] or [dykain] are not very common.

Word initial [y] is interchangeable with [o] but not with [a]. If there is a word starting with [y] there is nearly always also a similar word starting with [o]:
oaiin (26) daiin  (863) odaiin (60) okaiin (212) otaiin (154)
oain  (11) dain   (211) odain  (18) okain  (144) otain  ( 96)
oaiir ( 4) daiir  ( 23) odaiir ( 2) okaiir (  6) otaiir (  4)                                
oair  ( 3) dair   (106) odair  ( 5) okair  ( 22) otair  ( 21)
oar   (16) dar    (318) odar   (24) okar   (129) otar   (141)
oal   ( 3) dal    (253) odal   (13) okal   (138) otal   (143)
oam   ( 1) dam    ( 98) odam   ( 6) okam   ( 26) otam   ( 47)                 
oor   ( 3) dor    ( 73) odor   ( 8) okor   ( 34) otor   ( 46)            
ool   (--) dol    (117) odol   ( 2) okol   ( 82) otol   ( 86)
oy    ( 6) dy     (270) ody    (46) oky    (102) oty    (115)
yaiin ( 6)              ydaiin (21) ykaiin ( 45) ytaiin ( 43)
yain  (--)              ydain  ( 5) ykain  ( 10) ytain  ( 13)
yaiir ( 2)              ydaiir (--) ykaiir (  2) ytaiir (  1)
yair  ( 2)              ydair  ( 2) ykair  (  8) ytair  (  3)
yar   ( 2)              ydar   ( 2) ykar   ( 36) ytar   ( 26)
yal   ( 1)              ydal   ( 3) ykal   ( 16) ytal   ( 19)
yam   (--)              ydam   ( 1) ykam   (  5) ytam   ( 13)                     
yor   ( 2)              ydor   ( 1) ykor   ( 10) ytor   ( 14)
yol   ( 2)              ydol   (--) ykol   ( 14) ytol   ( 12)
yy    ( 1)              ydy    ( 8) yky    ( 18) yty    ( 24)

Of course, this proposed replacement scheme doesn't work for mid-word -a- or mid-word -o- instances at all, which is why it falls under the category of "orthographic", so y can be explained away as a scribal variant. :-) 

But given that so many words end in -dy, are you really saying that -dy is a replacement for -do or -da? I'm not sure you've really thought that through to the end line, sorry. :-(

My interpretation of the network of similar words is that word final [y] is a replacement for word final [a] and [o]. Word final [a] is nearly always replaced with [y]. Therefore a word final [a] is rare. Word final [o] did occur in only 314 out of 6920 word types (4.5 %).
ar (350) dar (318) odar (24) okar (129) qokar (152) kar (52) sar ( 84) rar (21) orar (10) olkar (19) lkar (30)
al (260) dal (253) odal (13) okal (138) qokal (191) kal (23) sal ( 55) ral (17) oral (10) olkal (11) lkal ( 5)
or (363) dor ( 73) odor ( 8) okor ( 34) qokor ( 36) kor (26) sor ( 57) ror (17) oror ( 5) olkor ( 4) lkor ( 4) lkr ( -)
ol (537) dol (117) odol ( 2) okol ( 82) qokol (104) kol (37) sol ( 75) rol (20) orol (15) olkol ( 5) lkol ( 5) lkl ( 9)
y  (151) dy  (270) ody  (46) oky  (102) qoky  (147) ky  (25) sy  ( 35) ry  (13) ory  (17) olky  (22) lky  (17) lk  ( 1)
o  ( 81) do  ( 16) odo  (--) oko  (  8) qoko  (  9) ko  ( 2) so  (  5) ro  (10) oro  ( 5) olko  ( 1) lko  ( 3)
a  (  3) da  (  9) oda  (--) oka  (  1) qoka  (---) ka  ( 1) sa  (  4) ra  (--) ora  (--) olka  (--) lka  (--)