Q. In the annals of Sanaatana Dharma why isn't there One Absolute Reality? Every "seer of truth" saw/interpreted the way they saw it.
Absolute Reality is only one but it is expressed differently because it can not be the topic/subject of mind or intellect!
Read the following Shruti quotes that say how it is beyond the of mind or intellect:
न तत्र चक्षुर्गच्छति न वाग्गच्छति नो मनः ।
न विद्मो न विजानीमो यथैतदनुशिष्यात् ॥ ३॥
अन्यदेव तद्विदितादथो अविदितादधि ।
इति शुश्रुम पूर्वेषां ये नस्तद्व्याचचक्षिरे ॥ ४॥
The eye does not go thither, nor speech, nor the mind. We do not know It; we do not understand how anyone can teach It. It is different from the known; It is above the unknown. Thus we have heard from the preceptors of old who taught It to us.
यतो वाचो निवर्तन्ते । अप्राप्य मनसा सह ।
आनन्दं ब्रह्मणो विद्वान् ।
न बिभेति कुतश्चनेति ।
He who knows the Bliss of Brahman, whence words together with the mind turn away, unable to reach It—he is not afraid of anything whatsoever.
Also Neti Neti (नेति नेति) clearly refers to it.
The difference in absolute reality you recognise is actually the difference in expression/description of it not the absolute reality itself! And as far as I can say the reason for why it is being expressed differently is due to the incapability of mind and intellect! The mind and intellect of everyone thinks, understand and interprets differently. Understanding own mind is also believed very hard.
Let me give an idea about such possible problem behind it: First of all absolute reality can not be exactly/absolutely described - 1st problem and if tried to describe into words, different people understand and interpret that described absolute reality differently - 2nd problem.
From above discussion, I hope you can understand why we recognise different absolute reality.
But don't get worried, the above problem is up-to mind and intellect and we're not limited to mind and intellect (मनोबुद्धयहंकार चित्तानि नाहं), when one realize the absolute reality it is the only one, true, absolute and ultimate reality!
Q. If there is One Absolute Reality and if there is one absolute path (Advaita/Visistadvaita, Davita, Achintya Bhedaabheda etc.), why is it that Vyasa Bhagavan did not state it conclusively in his Puranas?. Shaivaite scriptures portray Lord Shiva as Absolute, Vaishnava scriptures portray Lord Vishnu as Absolute and so on.
Yes there is only one Absolute Reality but there are many paths (there is not a strict path need to be followed) though they are likely to be merge finally! and talking about different Vedanta philosophy (i.e Advaita, Vishishtadvaita etc.), they are attempt/effort to conclude the absolute reality rather than paths! The paths are various practices or methods to do उपासना or साधना for attaining the absolute reality.
Read the following Smriti quotes that justify that the absolute reality is only one and different path unite to one destination!
From Shiva Mahimna Stotra:
त्रयी साङ्ख्यं योगः पशुपतिमतं वैष्णवमिति
प्रभिन्ने प्रस्थाने परमिदमदः पथ्यमिति च ।
रुचीनां वैचित्र्यादृजुकुटिल नानापथजुषां
नृणामेको गम्यस्त्वमसि पयसामर्णव इव ॥ ७॥
The different practices based on the three Vedas, SaMkhya, Yoga, Pashupata-mata, VaishhNava-mata etc . are but different paths (to reach to the Greatest Truth) and people on account of their different aptitude choose from them whatever they think best and deserved to be accepted . But as the sea is the final resting place for all types of streams , You are the only reaching place for all people whichever path,straight or zigzag, they may accept .
From Sarva Vedanta Sidhaanta Saar Sangrah:
ततस्तत्संबभूवासौ यद्गिरामप्यगोचरम् ।
यच्छुन्यवादिनां शून्यं ब्रह्म ब्रह्मविदां च यत् ॥ ९८०॥
विज्ञानं विज्ञानविदां मलानां च मलात्मकम् ।
पुरुषः सांख्यदृष्टीनामीश्वरोयोगवादिनाम् ॥ ९८१॥
शिवः शिवगमस्थानां कालः कालैकवादिनाम् ।
यत्सर्वशास्त्रसिद्धान्तं यत्सर्वहृदयानुगम् ।
यत्सर्वं सर्वगं वस्तु तत्तत्त्वंतदसौ स्मितः ॥ ९८२॥
And thereby the Yogi gets which is (called by) Agochara of Vaani, which is Sunya of Sunyavadi and Brahman of Brahmavadi,Vijnana of Vijnanavadis, Mala swarupa of Malachittas, Purush of the Sankhya Sastra and Ishwara of the Yoga Sastra, Shiva for followers of Shaiva Agama, Kaala for Kaalavadis, who is principle of every doctrine, who is situated in hearts of everyone, who is omnipresent, who is the true reality; that Yogi in the Videhamukta state establishes himself in the same state
According to Prasthana Bheda of Madhusudan Sarasvati,
सर्वेषां प्रस्थानकर्तॄणां मुनीनां
विवर्तवादपर्यवसानेनाद्वितीये परमेश्वर एव प्रतिपाद्ये
तात्पर्यम् । न हि ते मुनयो भ्रान्ताः सर्वज्ञत्वात्तेषां ।
किं तु बहिर्विषयप्रवणानापाततः पुरुषार्थे प्रवेषो न
संभवतीति नास्तिक्यवारणाय तैः प्रकारभेदाः प्रदर्शिताः ।
तत्र तेषां तात्पर्यमबुद्ध्वा वेदविरुद्धेऽप्यर्थे
तात्पर्यमुत्प्रेक्ष्यमाणास्तन्मतमेवोपादेयत्वेन गृह्णन्तो जना
नानापथजुषो भवन्तीति सर्वमनवद्यम् ॥
The views of all the munis who are the exponents of the prasthAnas
culminate in vivartavAda and their ultimate purport is in the non-dual
supreme Being who is the subject-matter of all the prasthAnas. Those
munis were not deluded; they were omniscient (and knew the ultimate
truth).. But, since people whose minds are engrossed in worldly objects
cannot be directly initiated into the supreme human goal, and in order to
prevent their succumbing to crass materialism, the munis propounded
different methods to suit the capacity and inclination of different classes of
people. Without knowing their real intention, those who assume that
meanings which are contrary to the true import of the Vedas were what
were intended by them, and take those alone to be what are meant to be
accepted, follow various paths. Thus everything is clear.
So, Absoulte Reality is only one, absolute and ultimate but it is expressed differently. No matter the different ways (mentioned in question) you follow, (if) you'll attain (it will be) only one absolute and ultimate reality!