Namaste Ravilochan - Nice attempt to divide the Smartas - keep it up.
Of course, advaitins who have read the classics know as a matter of fact Sri Shankara's preference for Vishnu. Atleast Krishna was his Ishta devata, if nothing else that prompted him to use Vishnu wherver he needed a name for Saguna Brahman. That amounts to nothing however.
You could consider Vishnu as the full SB if desired so. So what is Shiva and other Gods according to advaita? The worst case, Shiva being part of the SB (manifestation of Vishnu). But SB by definition is infinite and not finite. Shiva then is no finite part of this SB. Shiva is also infinite. How do you now compare Vishnu and Shiva -it is a comparison between two infinities, one bigger than the other? It is a pointless and worthless comparison for advaitins. No spiritual benefit results from such comparisons.
An advaitin will consider all the supreme dieites of Vishnu, Lakshmi, Shiva, Parvati, Brahma, Saraswati, Ganesha, Kartikeya, Shasta etc on an equal rank only and would not waste time in grading them even if they were technically different. Advaitins will differentiate them from dieties based on avidya - like Gramadevatas, Karmadevatas or Durdevatas.
Whervever Sri Shankara says that Krishna alone is the greatest or worthy of worship, ot naturally includes all of the above mentioned dieties, which are a direct and equal manifestation of SB. Infact, I doubt if many advaitins would agree with your assessment of Krishna as SB. Only very few of the advaitins, those geared for some kind of debate, would ever make a distinction between SB and NB. They are one and the same. If you give a name, form and quality to the NB which is beyond such things, it is SB. A full blown SB could be called Vishnu according to advaita.