whether a Sthitaprajna can be called Isvara of yoga philosophy as per Patanjali definition of Isvara.
No , Sthitapragya can not be called as Isvara of yoga philosophy as per Patanjali's definition of Isvara. Ishwara is a special kind of Being who was there time immortal.
क्लेश कर्म विपाकाशयैःपरामृष्टः पुरुषविशेष ईश्वरः ॥२४॥
kleśa karma vipāka-āśayaiḥ-aparāmṛṣṭaḥ puruṣa-viśeṣa īśvaraḥ ||24||
Ishwara is a special kind of Being, untouched by ignorance and the
products of ignorance, not subject to karmas or saṃskāras or the
results of action.
This point is cleared in the commentary provided by the author to the sutra. -
The Sthitapragyan (who does karma as per dharma, but unaffected by its result generated by karma) and those jivas who are "Mukta or Free" although are in such state presently , were not in this state previously. If they are born here on this earth and although got libration or capable of not getting affected by karma-Phalas that mean they were not as such in their previous lives.
But Ishvara was ever free from all the this and there is no relationship of Karmas and karma-Phalas etc. etc. with Ishvara , because he is not associated with any of this nor he will be and not before . i.e. they can not affect Ishvara at any time. He alone, being timeless, was present before teachers began.
Man is subject to the laws of birth and death, the laws of karma.
Ishwara is unborn, undying. Man is subject to his samskaras—the deeply
rooted tendencies which drive him on to further actions and desires.
Ishwara is free from samskaras and desires. Man, it is true, may
become liberated. But, even in this, he differs from Ishwara—for
Ishwara was never in bondage. After liberation, man is one with
Brahman. But he can never become one with Ishwara. (Indeed, the desire
to become Ishwara, the Ruler of the universe, would be the most insane
of all egotistical desires
This state of Sthitaprajna is called as vairagya in Yoga-Sutras.
And here is the Hindi commentary on above sutra.
This Isvara, as per yoga philosophy, is saguna brahman or nirguna brahman?
The Ishvara in the Yoga philosophy is both Saguna as Well Nirguna. In the Yoga Sutras of Patanjai meaning of ishvara can be interpreted in various ways. Ishvara can be theistic or one god , non-theistic i.e. Naturalistic , Humanism and one can choose a personal god to do dhyana ,Naam Japa etc. i.e. Saguna Ishavara ( Rama ,Krishna ,Shiva, etc.), or simply focus on the idea of the higher Self, higher Consciousness, Light Or Nirguna Brahman. Like "OM ॐ" i.e. Pranava.
तस्य वाचकः प्रणवः ॥२७॥
tasya vācakaḥ praṇavaḥ || Samadhi Pada Sutra 27||
The word which express Him is OM.
स्वाध्यायादिष्टदेवतासम्प्रयोग ||Sadhana Pada - Sutra 44 ||
svādhyāyād-iṣṭa-devatā saṃprayogaḥ ||44||
As the result of study, one obtains the vision of that aspect of God
which one has chosen to worship.
Bhakti yoga is the path of loving devotion to God. It is expressed by
means of ritual worship, prayer and japa. It is the cultivation of a
direct, intense personal relationship between worshiper and worshiped.
In the practice of bhakti yoga, some special aspect of God, or some
divine incarnation, is chosen, so that the devotee's love may become
more easily concentrated
From the above two sutras it's clear that the god in the Yoga Philosophy is both personal as well as impersonal. ( Saguna -Nirguna) .
Patanjali has described relation between Atma and the world based on "Samkhya Philosophy" of kapila and he accepted the 25 Tattvas or elements like kapila. But unlike kapila he introduced another element (which is of 26 th element) called Ishavara Vishesha i.e. ishvara. And thus he accepted the existence of Ishvara unlike Samkhya.
According to Samkhya Philosophy Atma=Purusha and this atma or purusha is pure consciousness . So according to Yog-Darshana by Patanjali this Ishawara is not Atma , but very special from this Atma or jivatma as per Advaita .
The Puruṣa is considered as the conscious principle, a passive enjoyer (bhokta).Ishvara is a distinct purusha unaffected by the causes of affliction and the effects of ripening stored karma.
Conclusion - This Ishvara concept of Patanjali of is neither a
creator God nor the universal Absolute of Advaita Vedanta school of
Hinduism.God in the Yoga Philosophy is both personal as well as
impersonal. ( Saguna -Nirguna) . And Sthitapragyan can not be called
as Ishvara because it is above all.Patanjali reasons that
the teacher of the first teacher can only have been God, since He
alone, being timeless, was present before teachers began.