According to the commentary, the world is kaaryam. The entire kaarya prapancha is 'sarvam'. The second sarvam in the compound word 'sarva-sarveshvarah' denotes the upaadaana kaaraNam. The Bh.Gi.11th chapter verse 'sarvam samaapnoshi tato'si sarvah' is cited and Vishnu is shown as the upadana kaaranam for the jagat. The word 'Ishvara' in the compound word is explained as the Turiya, the one that transcends the kaarya and kaarana prapancha. We get to see a fine allusion to the Mandukya Upanishad. The mantras before the 6th denote the kaaryam. The 6th mantra the Karanam and 7th mantra the Turiya. One gets to read a lot of Vedantic ideas in Nilakanthas commentary, both for the Mahabharata and for the Harivamsha.
In the Harivamsha, Vishnu parva, there is a stuti of Kshyapa Muni addressed to Shiva. This stuti is available here; http://mahabharata-resources.org/harivamsa/vishnuparva/hv_2_072.html Here is a verse यल्लिङ्गाङ्कं त्र्यम्बकः सर्वमीशो भगलिङ्गाङ्कं यद्ध्युमा सर्वधात्री ।
नान्यत् तृतीयं जगतीहास्ति किंचिन्महादेवात् सर्वसर्वेश्वरोऽसौ ।। 2.72.६० ।। All bodies in the world with masculine gender are forms of three eyed lord shiva. All bodies in the world with feminine gender are forms of universal mother uma. There is nothing else in the world other than these two. Lord shiva is the lord of all. Nilakantha, the Advaitin commentator says for this verse: 2-72-60 yaditi | yathA kAryaM sarvaM strIpuMsarUpaM bhagali~NgA~Ngam | evaM kAraNamapi tasya tadanurUpaM bhagali~NgA~NgaM bhagali~NgasaMyogarUpapratimAtmakam avagantavyam | tena yadidaM dR^ishyate loke shivaM li~NgaM tadabhimAni chetanodbhavamidaM brahmANDamityarthaH | ata evAsau sarvasarveshvaraH | sarvaM avisheShAtkR^itsnaH prapa~nchaH sarvaM samApnoShi tato.asi sarvaH iti nirvachanAttadupAdAnabhUto viShNurapi sarvashabdArthaH tayorIshvarasturIyabrahmarUpa ityarthaH |