From where does one coming from Jazan for Umrah on Saturday enter into a state of Ihram: from the Miqat of Yalamlam on Thursday, or from Abyar Ali on Saturday?

The most correct view is that you will first pass by your original Miqat (Yalamlam), then by a secondary Miqat (Dhul-Hulayfah). It is more cautious to enter into Ihram from the first Miqat. Nevertheless, there is no harm in delaying the Ihram until Dhul-Hulayfah, especially since Dhul-Hulayfah is further from Makkah than Yalamlam, and because you intend to go to Madinah first. Sheikh Ibn Uthaymeen issued a fatwa permitting the delaying of Ihram for one who intends to go to Madinah first and to enter into Ihram from its Miqat.