To be fair, Urmian has a more authentic Assyrian sound to it, considering that many Assyrians would typically have a dialect similar to Urmian ---- Iraqi Koine, Jelu and Nochiya dialects are a lot closer to Urmian. Tyari is like a "lovechild" of Urmian and Chaldean (as it uses the Th sounds). You can say that Chaldean is a concoction of Western Assyrian (Turoyo) and Assyrian Neo-Aramaic, although it still sounds more Eastern. As in, it will be more intelligible to us than to Western Assyrians.
No, Urmian is one of the least
"authentic"-sounding dialects. The only reason it's even vaguely considered as a "standard" is because, in the 19th century, Justin Perkins, an American missionary, came over and arbitrarily chose Urmian to be the language in which the Bible was going to be translated for all Assyrian Neo-Aramaic speakers, even though most of the Neo-Aramaic speakers at the time in Hakkari and the Nineveh Plains spoke dialects that were a lot more conservative. Before then, the modern dialects were almost never used in writing (for centuries, that honour would go to Classical Syriac).
Tyari is not the "lovechild" of anything---it preserves
the "th" sounds from 2500+ years ago from the days of Imperial Aramaic, as do the Chaldean, Turoyo, and Classical Syriac dialects. Urmian, Jilu and Nochiya lost
those sounds some time within the last few centuries (the dialects are related and probably branched off the same dialect or influenced each other due to proximity).
Btw, like Urmian, Chaldean is also a dialect with a foreign sound -- In this case, Arabic. So it's pretty much not any "better" than Urmian.
It depends on the specific dialect within Chaldean, there is no "one" Chaldean dialect any more than there is "one" Nestorian dialect. The Alqosh dialect, for example, has a lot
less Arabic loanwords than, say, the Telsqop dialect.