History of Lindiga sounds

First, a note on spelling. Since I'm thinking of creating a new alphabet
for Lindiga, not based on the Latin alphabet, I was briefly
experimenting with using IPA for all the Lindiga documentation. But
switching between the Latin and IPA keyboards is a little inconvenient.
I could add the necessary IPA characters to the Latin keyboard layout,
but I might as well just use accented Latin alphabet characters.

For the retroflex sounds, I could use the dot below characters (ṭ ṇ ṣ ḷ)
or some other accent below the character (ţ ņ ş ļ). There's that
retroflex lateral fricative [ɬ̢], though, which complicates things. (ł̣ ?
ł̧ ?) (Even the IPA [ɬ̢] is non-optimal, but it will have to do.) For now
I'll use digraphs with the letter -ř indicating a retroflex sound.

The regular lateral fricative is written "ł", as in Minza. The velar
nasal is "ng", which is unambiguous since [ɡ] is an allophone of /k/;
[ŋɡ] is /nk/.

As for the vowels:

i /i/  y /ʉ/  u /u/
ė /e/
e /ɛ/  ö /ɞ/  o /ɔ/
a /a/

with a breve for short unstressed vowels or semivowels (ĭ y̆ ŭ ĕ), where
ĕ represents [ə].

So ... now I need to figure out where these sounds came from. Lindiga
has three centering diphthongs (iĕ yĕ uĕ). Originally these may have
been long vowels, possibly [eː ɵː oː]. Other characteristic diphthongs
in Lindiga are the rising diphthongs (ei öy ou). One clue is the
alternation of iĕ with öy, as in the word for "eye".

viĕn (singular)
vöyn (plural)

If we assume some form of umlaut-type change affecting the vowel in
words like this, it's possible to imagine how these diphthongs came about.

ven-e -> veen -> viĕn
ven-u -> vöun -> vöyn

What about the retroflex sounds? Considering how rare a retroflex
lateral fricative is, it's likely that it came from some more complex
sequence of sounds, possibly a lateral fricative followed by a retroflex
approximant or fricative. I imagine the history of a word like "łřiĕsix"
(to continue, proceed) might have gone something like this:

[xləɽɛsix] -> [ɬɻeːzix] -> [ɬ̢iəʒiç]