Where are special jihad units?
There are none specifically relating to the jihad game mechanic. The Ghurid's have lots of Ghazi's and most Muslim factions can recruit Mutatawwia warriors along the silk road as well as Askari nobles -- jihad supporters.
It would be nice though if a general under the banner of Jihad, had a special mercenary pool to access though.
Will the Mongols be playable?
Not without a late start date script I think.
Will the Sultanate of Delhi be playable?
Yes, just look here.
Why are the answars only recrutable in the Oman armies?
Faction unit diversity? the rationale is touched on in the unit card, giving the background for those particular Ansars of the time period 1174+