I noticed that Scheherazade New is missing a very important and common ligature, one that occurs very frequently in the Quranic text. It is Lam-Alef, but with a high hamza located in the center, and with a fatha over hamza.
A number of other fonts support it, such as Coranica (contains special signs for the spellings of the Cairo print edition, the first proper printed version of Quran) and the Hafs Uthmanic Script font designed by the Saudi department for Quran publishing (although the later is not purely Unicode-based).
Without this ligature, it is impossible to write the Quran in its published form in a text. The ligature is used in several words in the Quran (middle eastern editions).
High hamza is a spacing character. As such, it would break the ligature, and so it will probably not work well with any font. It may be that you just need a specialized font that uses the regular hamza with a different attachment.
The formal Unicode way to represent this sequence is Lam, Tatweel, Hamza Above, Alef. Fonts can ligate this sequence to create the special lam-alef ligature with hamza in the middle.
