OK ! No Problem at all !!
(All the queries in the question seems like complaining about a spiritual organisation. The findings are like misinterpretions about any religious belief/sect.)
The Chhandogyopanishada tells about expansions of eternity which is descending from the One Supreme towards the material existence of five elements, "eko aham bahu syam".
तत इक्षत बहु स्यां प्रजायेयेति तत्तेजोऽसृजत तत्तेज ऐक्षत बहु स्यां प्रजायेयेति तदपोऽसृजत | तस्माद्यत्र क्वच शोचति स्वेदते वा पुरुषस्तेजस एव तदध्यापो जायन्ते || (६.२.३)
"Tat ikshata bahu syaam prajaayeyeti tattejosrijata tatteja aiksata bahu syaaṃ prajaayeyeti tadaposrijata tasmaadyatra kvaca shochati svedate va puruṣastejasa eva tadadhyaapo jaayante"(6.2.3)
That Existence decided: ‘I shall be many. I shall manifest Myself.’ He then created fire. That fire also decided: ‘I shall be many. I shall be born.’ Then fire generated water. That is why whenever or wherever a person mourns or perspires, s/he produces water.
Now, let's come to the original point of the eternity and its descending transcendence within the eternal existence.
Rama as Radhika-Raman is Krishna Himself. And otherwise, Balarama, Raja Ramachandra and Parshu Rama are Lilavatars, eternal manifestations of Krishna, with pastimes in mundane worlds, through Vishnu.
Similarly, Anant-Vishnu* is Krishna Himself. And other Vishnu forms (Karanodsayi, Garbodsayi and kshirodsayi) are eternal personality expansions or Purushavataras of Krishna thru Sankarshan & Narayan¤.
By the way, the ISKCON uses in the literature some phrases, focusing at western world audience who have greek-roman civilization rooted educational background, the Big-demigods for the Mahadevas and demigods for devas like agni, vayu. Infact, some deep studies of the Srimad Bhagvatam also revealed that the mahadevas are by the eternal existence, for the material control and the devas are by the material existence, for the material existence, so the names.
Nevertheless, Sanskrit root "maha(t)" literally means big in English. Correct!! And no harm in calling devas like indra, agni, vayu et al as demigods, isn't it. Interesting dogma here is, we easily accept these devas to be called as demigods but always dislike Mahadevas been called as Big-demigods. And also nothing like so-called un-Vedic here, so far as interpreting on a basis of the Sanskrit grammar.
And about ISKCON and other Gaudiyas; they follow their respective acharyas (spiritual masters) and not Lord Chaitanya, Who is accepted as Krishna Himself, in the mood of Radha's devotional ecstasy. Infact, nobody is capable of following what Lord Chaitanya did during His Pastimes. The ISKCON and the Gaudiyas, however, follow eight-fold instructions/teachings compiled by Lord Chaitanya Himself, and take help of the example set by their acharyas. (http://www.harekrsna.de/Siksastaka/Siksastakam-E.htm)
Yes. At all the ISKCON/Gaudiya temples in the morning prayers (mangala arti) Tulsi arti is sung as "vishnu bhakti pradde devi, satyavatiyye namo namah..". So, Vishnu (Krishna) bhakti is granted/fortified by Tulsi devi. In the Juhu (Mumbai) ISKCON temple especially, Ram-Laxman-Sita deity is worshipped. But, who told you Krishna is worshipped as ishta in ISKCON. The ISKCON and/or the other Gaudiyas have no such system of ishta.
Moreover, the ISKCON literature depicts that; normally in the so-called Hindu societies the concept of worshipping ishta or ishta-devta was introduced to convince people involved in polytheism (mutiple gods theory) to worship impersonal Brahman by selecting only one personal form or ishta-devta. Especially, so because the impersonalists have made a philosophy that by becoming devoted to any one of them you will attain Brahman. So they call these deities as ishta devata. Perhaps, inspired by the dramatic success of foreign sophistry based religious literature, the "ishta" system was embedded in some Indian scripture, afterwards in the medieval times. In modern days, we find them propagating concept of "ishta-devta" as analogous to a microphone which amplifies the meditational voice to their impersonal God.
Anyways, asking/mooting such points is considered as good as negating Krishna's teachings in the B.Gita chapter #7 & 12. When we have asked the ISKCON practicing devotees about the same, they first denied commenting on such enquiries, based on impersonal monotheism. They said the Bhagavatam (1.3.28) clears that "Krishnastu Bhagvan Svayam". So, the ISKCON philosophy of monotheism respects every servant of Bhagvan, equally They follow the spiritual instructions based on the B.Gita and the Bhagvatam.
That is further explained as;
in spiritual and material domains, any work (action) is done for some cause. As a slave is for the master; the action, "karyam" is for a "kaaranam", a cause. And "kaarnam bina karyam kadachit na vidyate", and that's why there is no work (action) without its cause. The Svetasvatar Upnishada (6.8) says God only exists beyond this cause-action relationship, "na tasya kāryaṃ karaṇaṃ ca vidyate ..". He is the supreme master and supreme cause of every cause, eternal and mundane, but He Himself has no cause. He is the supreme cause of His personal and impersonal causes too (Bh.Gita #14.27).
*The Bhagavatam, the eternal Gopis sing His glories as "Anant-Vishnu" in His Infinite transcendental characteristics but they reject Krishna as "Purna" Narayana in His complete features manifestation, at their supreme exalted stage of devotion. This we also find in famous prayers of the "Damodar-ashtakam" by Satyavrata Muni. Here, "Ananta-Vishnu" should not to be confused with the deity "anantasesha-vishnu".
¤The Ishopanishada and other Vedic Upnishadas salute Mahavishnu and Narayan as "Purna" or complete, because of possessing all the six spiritual opulences. Krishna, however, is beyond these complete divine features and is infinite in any sense of divinity. Krishna is the only real "kartum-akartum-anyathakartum" samartha.