"Omar has an Omani door."
Translation:عِنْد عُمَر باب عُمانِيّ.
2a is just a vowel following a glottal stop, so it should be just like starting a vowel from silence (ie. like the a in arm, and the e in exit) - or at least that's how I was taught.
It's a little more complicated when ء is in the middle of a word. You should just try to close your throat, and afterwards you pronounce the vowel.
3a I would describe like moaning with your throat closed.
Someone correct me if I'm wrong, please.
On the keyboard I am using, the diacritics (the marks) are situated on the first and second row. They can be typed by holding Shift key. The diacritic for doubling a consonant ( ّ ) is found in the top left corner, also hold Shift key to type it. See also picture of keyboard here: https://i.pinimg.com/originals/d4/15/3c/d4153c8f8caa2a3d60e3f26f0b5c6d08.png