Proto-Tirelat and sound changes

Shirahlat is so far very close to Czirehlat except for a few changes to the
phonology. I've restored the vowels "ü" [y] and "ö" [ø], and I'm
considering adding the consonants "th" [T] and "dh" [D]. I've been avoiding
these sounds since I tended to overuse them in my older languages, but it's
quite possible the weasel folk picked them up from English. I've also
czancged the spelling to be more readable, since I'm planning on creating a
real alphabet for Shirahlat, and there's no need to use a potentially
confusing romanization.

After a few attempts to convert Czirehlat to Shirahlat directly, I thought
it might be better to do a Proto-Tirelat phonology and have rules to go
from PT to both languages. Generally speaking, Tirelat/Shirahlat /ü/
corresponds to /ui/ or /iu/ in Czirehlat, and /ö/ corresponds to /ue/ or
/io/. So there could be a number of ways this situation could be derived
from Proto-Tirelat:

1. Original /wi/, /yu/ > /ü/; /we/, /yo/ > /ö/ in Shirahlat
2. Original /ü/, /ö/ diverged in Czirehlat, depending on surrounding sounds
3. Some combination of these.

The origin of /T/ and /D/ could either have come from English influence
(replacing some older Tirelat sound) or it could have been an original
Tirelat sound that was lost in Czirehlat. I'm leaning towards /T/ and /D/
being original Tirelat sounds, which largely merged with /f/ and /v/ in
most contexts in Czirehlat. Perhaps Czirehlat "hl" is from an original
/Tl/, and "l" from /Dl/. In this case, Shirahlat might turn into
"Shirathlat".

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