Since you believe the Stotrams attributed to Adi Shankara are later works, I guess then it will be difficult to prove that he believed all the Panchatayana deities as Saguna Brahman. This is because his major works only refer to Sriman Narayana as supreme alone.
For example, his Bhagavad Gita commentary states that nobody is equal to Sriman Narayana:
Asi, You are; pita, the Father, the Progenitor;
lokasya, off all beings; cara-acarasya, moving and
nonmoving. Not only are Yur are Father of this world, You are also pujyah, worthy of worship;
since You are the guruh, Teacher; [He is the
Teacher since He introduce the line of teachers of
what is virtue and vice, and of the knowledge of
the Self. And He is greater than a teacher because
He is the teacher even of Hiranyagarbha and
others.] gariyan, greater (than a teacher). How are
You greater? In answer he says: Asti, there is; na,
none other; tvat-samah, equal to You; for there is
no possibility of two Gods. Because all dealings
will come to naught if there be many Gods! When
there is no possibility of another being equal
toYou, kutah eva, how at all; can there be anyah,
anyone; abhyadhikah, greater; api, even; loka-
traye, in all the three worlds; apratima-prabhavah,
O you of unrivalled power? That by which
something is measured is pratima. You who have
no measure for Your power (prabhava) are a
pratima-prabhavah. Apratima-prabhava means 'O
You of limitless power!' Since this is so, (Adi Shankara's commentary on Bhagavad Gita 11.43)
In Bhagavad Gita 9.25, he explicity states that worshipping other deities such as Vinayaka (Ganesha) is not the same as worshipping Vishnu:
Deva-vratah, votaries of the gods, those whose
religious observances [Making offerings and
presents, circumambulation, bowing down, etc.]
and devotion are directed to the gods; yanti, reach,
go to; devan, the gods. Pitr-vratah, the votaries of
the manes, those who are occupied with such rites
as obsequies etc., who are devoted to the manes; go
pitrn, to the manes such as Agnisvatta and others.
Bhutejyah, the Beings such as Vinayaka, the group
of Sixteen (divine) Mothers, the Four Sisters, and
others. And madyajinah, those who worship Me,
those who are given to worshipping Me, the
devotees of Visnu; reach mam, Me alone. Although
the effort (involved) is the same, still owing to ingorance they do not worship Me exclusively.
Thereby they attain lesser results. This is the
meaning. 'Not only do My devotees get the
everlasting result in the form of non-return (to this
world), but My worship also is easy.' How? (Adi Shankara's commentary on Bhagavad Gita 9.25)
However, I did find a few references where he says both Vishnu and Shiva are supreme.
From Prasnottara Ratna Malika:
कश्च भगवान् ?
Who is the Lord ?
महेशः शंकरनारायणात्मकः ।।
The Supreme Lord who is both Narayana and Shankara. (Prasnottara Ratna Malika, Verse 65)
In his commentary on the Vishnu Sahasranama, Adi Shankara quotes Shiva's statement in Harimvasa Parva 3.88 of the Mahabharata where he seems to be hinting at equality between Vishnu and Shiva:
नामानि तव गोविन्द यानि लोकेमहान्ति च ।
तान्येव मम नामानि नात्र कार्या विचारणा ।।
Oh Govinda your names which are famous in this world, these are also my names there is no doubt in it. (Adi Shankaracharya's commentary on the Vishnu Sahasranama)
Otherwise, most of his philosophical works refer to Sriman Narayana alone to be supreme.