Is it sufficient for a pilgrim who performed Umrah on behalf of his martyred brother to assume ihram for Umrah from Al-Aziziyah, or is it obligatory to assume ihram from Al-Taneem?

No specific individual can be declared a martyr, except for those whom the Messenger of Allah, peace and blessings be upon him, testified for. As for someone who has performed Hajj and wishes to perform Umrah afterwards, they should go to the nearest boundary of the Haram ( حل ), such as Tan'im or Ji'ranah, to enter into the state of Ihram for Umrah from there, as the Prophet, peace and blessings be upon him, commanded Abdur-Rahman ibn Abi Bakr to take Aisha to Tan'im for her to enter into Ihram for Umrah. If you enter into Ihram from within the Haram, your Umrah is valid, but you are required to offer a sacrifice (dam) to be distributed among the poor of the Haram.