History of Yasaro

In the interest of realism, I've been wanting to go more into the
histories of some of my languages, trace the etymologies of words, and
compare related languages. I'm starting with Yasaro, since it's not
closely related to the other languages, so I can more freely design
ancestor languages and pick sources for Yasaro words.

One thing I'm wondering is how many sound changes are typical in
languages over a historical period? In other words, what's an
approximate number of different historical sound changes that would have
occurred from Latin to the Romance languages, or Classical Chinese to
the modern Chinese languages?

The other thing is how far the meanings are likely to change, and what
percentage of the words are likely to be affected. Is it typical for
languages to lose the basic meaning of words like "head" and "fire" and
replace them with "pot" or "hearth" (as in the Romance languages) over a
relatively short period of time?

But for now I'll try to focus on sound changes. I started playing around
with the history and came up with a number of rules, but I'm not really
satisfied with them. I'm calling the ancestor of Yasaro "Alpha" for now,
and other languages Yasaro borrowed words from "Beta", "Gamma", and so
on. If I can't figure out an ancestral form that works with the sound
change rules, I can always explain it as a borrowing. Yasaro is further
divided into Old, Middle, and Modern versions, each with its own
phonology. Middle Yasaro is pretty much the current version of Yasaro as
I've been using in my internal documentation, with all nasal vowels
intact and the original positions of the stress before it shifted, but
with some additional diphthongs which turned to long vowels in the
modern language.

Between Middle Yasaro and Modern Yasaro, /a~/ and /u~/ merged to a
single phoneme, /o~/ (but both /a~/ and /u~/ are preserved in the
spelling). Before stops, a homorganic nasal was inserted after a nasal
vowel (/e~p/ > /e~mp/), and the vowel then lost its nasality (/e~mp/ >
/emp/). The question now is where did the nasal vowel come from in the
first place? Does it make sense for /em/ to turn into /e~/ at some
point, then go back to /em/? But I can't think of many other reasonable
sources for nasal vowels. Could a sequence like /kna/ turn into /ka~/?

The other thing I'm concerned about at the moment is the origin of /tS/.
Originally /k/ before /i/ seemed to be a reasonable starting point, but
how to explain occurrences of /ki/ in the modern language? Well, there
could have been another vowel /y/ or /1/, which merged with /i/ after
the /k/ > /tS/ | _/i/ change. But now I'm thinking something like /kr/,
/tr/, or even /pl/ could be a possible source of /tS/, and might be less
problematic (since these clusters don't exist in modern Yasaro).