I have mainly seen Adi Shankaracharya calling Narayana to be the only supreme/Saguna Brahman/Ishwara whereas his followers i.e. Smarthas believe that Adi Shankaracharya saw Vishnu, Shiva, Devi, Surya, Kartikeya and Ganesha as Saguna Brahman. Are there any instances in the works of Adi Shankaracharya where he clearly refers Lord Shiva, Mother Durga, Surya, Kartikeya and Ganesha as supreme/Saguna Brahman/Ishwara?
Adi Shankara didn't differentiate various forms of Saguna Brahman. He explicitly says that in Prasnottara Ratna Malika.
Adi Shankara at many places says Shiva, Vishnu, Amba(Supreme Goddess), Surya as Iswara and also uses words like Brahman, Paramesha, Mahesha in this work
65.कश्च भगवान् ?
Who is the Lord ?
महेशः शंकरनारायणात्मकः ।।
The Supreme Lord who is both Narayana and Shankara.
Adi Shankara says Divine Mother is Supreme Goddess.
54.का च परदेवता उक्ता?
Who is said to be Supreme Goddess?
The consciousness - power (Amba).
को जगद्धर्ता ?
Who is the protector of the world?
Adi Shankara says Knowledge can be attained from Shiva only.
को हि जगद्गुरुरुक्तः ?
Indeed, who is said to be the World-preceptor?
ज्ञानं कुतः ?
Whence knowledge ?
Only from Shiva.
He says liberation can be attained from devotion to Mukunda and when asked who is Mukunda, he says Mukunda is the one who takes us across ignorance (Advidya).
56.मुक्तिं लभेत कस्मात् ?
How can one get release ?
By devotion to Mukunda .
मुकुन्दः कः ?
Who is Mukunda ?
यस्तारयेत् अविद्याम् ।
One who takes us across ignorance.
का च अविद्या ?
And what is ignorance ?
यत् आत्मनो अस्फूतिः ।
The non-manifestation of the Self.
He says reality exists in Shiva, Gayatri and fire.
61.को ब्राह्मणैः उपास्यः?
Who is to be contemplated by the Brahmanas?
गायत्रि - अर्क:- अग्नि - गोचरः शंभुः ||
Siva who is seen in the Gayatri, Sun and fire.
गायत्र्यां आदित्ये च अग्नौ शंभौ च किं नु ?
What is there in the Gayatri, Sun, fire and Siva?
तत् तत्वम् ।
That is the reality.
He also uses words like Brahman, Paramesha to describe Supreme Lord.
58.को मायी ?
Who wields Maya?
The Supreme Lord (Paramesha).
सत्यमपि च किम् ?
And What is real?