The answer will be a bit longer as I have to tackle 4 questions in a single one.
I am gonna go in the reverse order.
Are there any mention of units bigger than the Mahakalpa (100 years of Brahmā ) also called, para (पर) i.e., Two Para-ardhas ?
I think, as each Vishnu and Rudra also has a life span of 100 years according to their own time calculation. We can assume that, there should be some higher kalpa also greater than mahakalpa as i have discussed in this answer of mine though I haven't came across any such names.
Which scriptures portray Brahmā (ब्रह्मा) on a par level (or maybe even greater than) with Vishnu and Shiva, (at least in terms of lifespan) ? Like Shiva and Vishnu are centrally glorified as the Para-Brahman in the Shiva Purana and the Vishnu Purana, respectively.
The only reference i can give is this.:
Sri Shiva Purana.:
Vayu said, I shall narrate how Rudra was born and how Brahma and Vishnu were born from each other. The three are Atmans as causes of born of Mahesvara; they are cause of creation, sustenance and annihilation of the Universe consisting of mobile and immobile beings. Endowed with great qualities they are sanctified by great Lord. Presided over by his Shakti, they can perform their activities. Brahma can create, Vishnu can protect and Rudra can annihilate. But they rivaled with one other. Desirous of excelling one other they propitiated their father the Supreme Lord by means of penance. Attaining all around favor of Lord at outset in Previous Kalpa, Rudra created Brahma and Vishnu. In another Kalpa, Brahma created Vishnu and Rudra. In another Kalpa, Vishnu created Brahma and Rudra. Thus in different Kalpas, Brahma, Vishnu and Rudra desiring mutual benefit are born of one another. Based on the events in their respective Kalpas, their process in being the cause of origin of one other is extolled by Sages. (Chapter 13, Vayavivasamhita, Shiva Purana).
Sri Skanda Purana 7.1.2.:
87-88-In the Sattvika Kalpas, the greatness of Hari is supreme. They know that the greatness of Brahma reigns supreme in the Rajasa Kalpas. Similarly, the greatness of Samhara Siva (Rudra) is to be found in the Tamasa Kalpas. In a Purana of a mixed nature, the greatness of Saraswati and Pitrs is narrated.
As, In some whatever Kalpa, Brahma creates Vishnu and Rudra and destroys them and is supreme in that Kalpa. So, his Life span must be greater than the both of them as he is the supreme one Param Brahman in that Kalpa.
As, I have discussed in the above links that TatParatPara Kalpa is a breath of Sada Shiva. Padma Purana Patala Khanda Chapter 108. says, that tri-gunas came from the 3 vedas and veda says that vedas are the breath of Bhagawan Shiva. So, his breaths varies as sat, tam, raj and so different kalpas.
If Lord Hanumān is supposed to be the next Brahmā, then Brahmā being the post ( pada ), his name will be Hanumān only, much like the name of our current Indra is Purandara. So does any other scripture mention any list of names of the Brahmās, of the past, the present and the future?
Also, if the proposition in question no. 1 is scripturally valid, what about the, any kind of mention of the names of Vishnu and Shiva for the past, present and future, is that found too?
Yes, the name of the present Brahma is Śatānanda.
Sri Skanda Purana 126.96.36.199.:
Lord Shiva Said -
Six Brahmās have passed away. The present Prajāpati who is well-known as Śatānanda is the seventh one, O goddess.
The names are as follows.: Viriñci, Padmabhū, Svayaṃbhū, Parameṣṭhin,
Hemagarbha (Hiranyagarbha), Śatānanda and the future one will be Caturvaktra.
Now, if you see these are all the names of Lord Brahma only. The concept is same for Lord Rudra as in 1st Kalpa Sadyojāta was born, next Vamdeva and so on and can be similar for Lord Vishnu also like 1st kalpa Janardana was born, next Vāsudeva and so on.
As, i have already discussed in this answer of mine that it's the same Brahma who takes birth again and again. And i think it's also same for Lord Indra also. As mentioned in this answer that as per Vishnu Purana to save themselves from Pralaya they entered into the womb of Aditi as 12 Adityas.
Well, there is one story in Mahabharata which was narrated by Krishna to Yudhishthira. In Udyoga Parva, Indra says it clearly that earlier he was capable of vanquishing Vritra, but this time he was weaker comparatively.
“sarvaṃ vyāptam idaṃ devā vṛtreṇa jagad avyayam | na hy asya sadṛśaṃ kiṃ cit pratighātāya yad bhavet | samartho hy abhavaṃ pūrvam asamartho 'smi sāṃpratam |” (MBH 5:10:1-2).
“Indra said, This whole indestructible universe, O gods, hath been pervaded by Vritra. There is nothing that can be equal to the task of opposing him. I was capable of yore, but now I am incapable”.
Indra is a self realised soul as I have discussed in this answer of mine.
So, MahaBali (Meaning the one with great strength) and Lord Hanuman, whom Sri Tulsidas Goswami ji on his Ram Charit Manas has described as the combination of Tridevs. I think that due to their good karma they will get Swaroopa Moksha with Lord Indra and Lord Brahma respectively.
Sri Shiva Gita Chapter 11.:
12-15.:Till the time the Jiva gets liberation, it keeps on taking births again and again. One who enjoys sinful deeds, would enter a body called 'Yatana Deham' (punishment body) after death and would undergo tortures in hell. One who had devoted himself to rites and rituals throughout his life, attains the Pitrulokam (abode of Pitris). One who had done sacrificial rituals, built wells etc. sacred deeds in his life, he first enters Dhoomam, then night, then Krushnapaksham, from there Dakshinayanam, after that goes to Pitrulokam and from there it attains a divine body and goes to the abode of moon.
16-20.: Together with the moon God, the Jiva enjoys there till his balance of Karmaphalam lasts, and after that again he comes back to Earth and takes birth. In the abode of moon, after his karmaphalam ends, he discards the Bhoga Shareeram (body of enjoyments) and wears the Linga deham again, and gains the attributes of sky, air, water, one by one and from that water becomes a cloud, then becomes rain and again transforms himself into the rice etc. food itemsand takes birth. In this few take the form of various creatures. Few born in the form of immobile creation. later, that food grains in the form of which this Jiva came to earth, get eaten by the creature (couple) and that Jiva who is in the form of food takes the form of Shukra, Shonita (male and female seeds) of the parents. later on the same couple unites their seeds give birth to this Jiva in gross form through the womb.
21-26.: Subsequently based on the Karma of the Jiva, female, male or eunuch bodies are obtained. Now I would explain the path of reaching higher abodes (gati) for the Jiva. Listen! One who possesses peaceful nature etc. good qualities , remains inclined towards the scriptures and righteousness, he is taken away till Brahma loka in celestial plane. With divine splendor, he becomes Devapurusha, and stays during Suklapaksham, Uttarayanam. First attains to the abodes of Sun and moon, then atains to Vidyullokam, after that a celestial deity comes descends from Brahma loka to Vidyulloka (IndraLoka) and takes the Jiva to Brahma loka.In the abode of Brahma with the celestial body the Jiva enjoys all his desires for a long time. After staying there for a long period he gets moksha along with Brahma (viz gets absorbed into Sadā Siva by getting Kaivalyam Moksham).
27-29.:Only that Brahmajnani remains one with Brahman as like as salt melts in water. As like as one can recall the dreams after waking up, a Brahmajnani also can recall everything similarly. For a Jiva who is devoid of Vidya (wisdom) and Karma third place is given which is Hell. Such a Jiva experiences extreme torture in Hells named raurava and similar ones and with the leftover Karma he gains birth as a heinous creature like flies, mosquitoes etc. on earth. this is about the Jiva Gati topic.
Thus, a soul gets Swaroops moksha and can enjoy the position of respective gods according to his karmas. If his kārmic balance gets over it again gets born as different creatures according to karma and if it's karmic balance remains intact it gets Kaivalya moksha becoming one with Brahman himself.
So, what is the scriptural (any kind) source & validity of this proposition?
You can find the scriptural basis in this answer of mine.
Conclusion.: As per my opinion and analysis, the Tridevs and Indra are eternal. One can attain that form by attaining moksha based on their karmas.
I hope this clarifies all your queries. Prd..