Did adi shankara believe that Brahman is all powerful or omnipotent?
Yes, (saguNa) brahman is considered omnipotent by Adi Shankara. He mentions this in Brahmasutra bhAshya 1.1.2.
अस्य जगतो नामरूपाभ्यां व्याकृतस्य अनेककर्तृभोक्तृसंयुक्तस्य प्रतिनियतदेशकालनिमित्तक्रियाफलाश्रयस्य मनसाप्यचिन्त्यरचनारूपस्य जन्मस्थितिभङ्गं यतः सर्वज्ञात्सर्वशक्त कारणाद्भवति, तद्ब्रह्मेति वाक्यशेषः।
That omniscient omnipotent cause from which proceed the origin, subsistence, and dissolution of this world--which world is differentiated by names and forms, contains many agents and enjoyers, is the abode of the fruits of actions, these fruits having their definite places, times, and causes, and the nature of whose arrangement cannot even be conceived by the mind,--that cause, we say, is Brahman.
Just to reiterate, Adi Shankara repeats the same at other places too. For example, just before the beginning of Brahmasutra bhAshya 1.1.5
एवं तावद्वेदान्तवाक्यानां ब्रह्मात्मावगतिप्रयोजनानां ब्रह्मात्मनि तात्पर्येण समन्वितानामन्तरेणापि कार्यानुप्रवेशं ब्रह्मणि पर्यवसानमुक्तम् । ब्रह्म च सर्वज्ञं सर्वशक्ति जगदुत्पत्तिस्थितिलयकारणमित्युक्तम् ।
So far it has been declared that the Vedânta-passages, whose purport is the comprehension of Brahman being the Self, and which have their object therein, refer exclusively to Brahman without any reference to actions. And it has further been shown that Brahman is the omniscient omnipotent cause of the origin, subsistence, and dissolution of the world.