Guttural-letters

Talmud says there are four guttural-letters: Aleph, Hei, Chet, Ayin.
Rambam and Shulchan-Aruch say that there must be done distinction between Aleph and Ayin letters (I think the same like Hei differs from Chet).
Why Ashkenazim don't pronounce gutturals?

In the past, many Ashkenazim pronounced the letter ayin as "ng". I have read that this was borrowed from the Dutch Sephardim, whose mesorah was Iberian in origin. In the present day, I have heard of individuals in Washington Heights who maintain this distinction, although it was apparently uncommon among the younger Frankfurter Jeckes during the Weimar Republic.

Today in modern Israel Hei and Aleph are not pronounced at all. Though Torah points Aleph with Dagesh: Bereshit 43:26, Vayiqra 23:17, Ezrah 8:18, Iyov 33:21. It means Aleph should be pronounced always and it should differ with or without Dagesh.

How to pronounce Aleph? Maybe Aleph must be pronounced like today Ayin, but Ayin must be pronounced little tongue must touch throat starting with g-sound like gAyin? Because alternative ancient pronounciation of עזה and עמרה sounded as gAzah and gAmorrah like Europeans say it Gaza?

Is it possible to assume that Ezrah-sofer removed guttural-sounds from Lashon-Qodesh? I heard that nations did magic only with guttural-sounds in ancient times and any magical words lost their strenght without gutturals.

I think almost all Israelis do pronounce the Hei.

I believe the consensus of medakdekim is that the dot in the alef is not a daggesh, but a mapik, put in by the chachmei hamesorah as a caution in places where one might be prone not to make a new syllable with the alef for whatever reason. An alef creates a new syllable and mayb should have a "glottal stop" (sorry to get too technical here), but can't really be called a guttural. This can be demonstrated, among other things, from the lack of patach genuvah.

The ayin as known from our mesorah has only one sound, but it does appear that at least some communities bizman bayis sheni and earlier pronounced one similar to a voiced khaf, which is why the Arabs say GHaza and the Greeks used a G when transliterating some ayins and not others. The Seridei Eish has a teshuvah touching on this when discussing the pasuk of Ufara es-Rosh Haisha. But even in this system, many roots with Ayin are not pronounced with a GH sound at all. And our mesorah doesn't hold by this anyway. The only letter we pronounce differently depending on the shoresh is shin and sin.

I would be quite surprised if Ezra removed any guttural sounds from lashon hakkodesh given that the biggest enemies of his, the Shomronim are b'davka the only Jews or quasi-Jews not to pronounce any guttural sounds- I believe they pronounce even the ches like an alef. בלאו הכי, many groups of Jews still pronounce a guttural ayin, ches, and g' refuyah.

Melech

Even if the Dutch Ashkenazi "ng" for Ayin was borrowed from the Dutch Sefardim, I would suspect that at least some Ashkenazim earlier had some kind of an N sound for ayin, as reflected in the Yiddish pronunciation (in certain dialects) of some words with an ayin, like Yankev.

Melech