Is זֹהַר accented on the first syllable? In other words, is it a segholate noun? I know segholate nouns are defined as those nouns that have three letters and are accented on the first one, e.g. סֵפֶר and מֶלֶךְ.
Drink: Is the reason why זהר zohar, despite being a segholate noun, has a פתח on the second letter that the second letter is a guttural? I think the same thing happened in שֵׁמַע shema, another segholate noun. It means "audio" or "renown".
The concept of a "segolate" was conceived of precisely for the purpose of describing this category of words. I don't know what else you want me to say.
It fits the exact pattern for a segolate with a guttural in this position. For more examples, compare צער, יחד, פחד, סהר, כעס, מחט, זעם, פעם, שער, לחץ, נחל, and that's just quickly off the top of my head.
That is not correct. In זוהר you have a long vowel. In שמע you have a long vowel. In מוות you have a long vowel. And in pausal position, the stressed segol (usually) or patach (always) becomes a qamatz.
The last vowel can be long only in the special case of the last root letter being י or ו, such as in חולי and תוהו.