we should make a pause (by reciting zikr) to separate fardh and sunnah salaat

1. The Hadith that you have made reference to in your question is reported in Sunan Abu Dawud (pg.144; Qadeemi). The Hadith includes an unqualified command by Umar [radhiallaahu anhu] to his companion to sit down. There is no instruction to make Dhikr as you have asserted. There is no contradiction between the purport of this Hadith and the official ruling of the Hanafi Madhab upon which Fatwa 13384 is based albeit that the Hadith in question is relatively weak with regard to its chain of narrators. Notwithstanding, the Hadith explicitly explains that it was the practise of the Ahlul-Kitaab that they would not separate between two Salaats at all. The ruling of the Hanafi Madhab is that separation between two Salaats should be established either through speech or action, e.g. moving right or left, etc. However, although the need to establish a separation exists, the need should not be extended beyond necessity. This ruling is based on the Hadith of Aaisha [radhiallaahu anha] reported by Imaam Muslim [ra] in his Sahih that Rasulullah [sallallaahu alayhi wasallam] would not sit after his salaam from Salaat except to the extent of ‘Allaahuma antas salaam…’ (vol.1 pg.218; Ilmiyah)

On a note of clarification, other words of Dhikr and du’aa have also been established from Nabi [sallallaahu alayhi wasallam] after Salaat other than those mentioned in the Hadith of Aaisha [radhiallaahu anha]. However, all these Dhikr and du’aas are all of similar length with slight variation. Therefore, the ruling of the Hanafi Madhab is not limited to the words in the Hadith of Aaisha [radhiallaahu anha], but may also be applied to other forms of Dhikr and du’aa. However, the end result is the same in terms of time. Furthermore, this ruling does not apply to one intending to perform his Sunnah or Nafl Salaat at home (after performing the Fardh in the Musjid). The superiority of these Salaats at home has been established from the Hadith of Nabi [sallallaahu alayhi wasallam]. (Musnad Ahmad vol.14 pg.342)

2. Wahdat al-Wujood according to the Ahlus sunnah Wal Jamaa is a concept explained by the mystics as when a person who is treading the path of reformation becomes so conscious of the existence and presence of Allah that he becomes oblivious to everything else. In all of the creation of Allah, he sees the greatness of Allah with the result that he becomes so overwhelmed with the greatness of Allah than the creation itself seems non-existent. This in no way negates the existence of the creation of Allah. By example, in a wresting contest between a professional wrestler and a child, the child seems non-existent. This does not negate the presence of the child. However, the overwhelming presence, strength and control of the wrestler makes one oblivious of the presence of the child. (Ahsanul Fataawa vol.1 pg.554; Karachi)

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai