I have heard the Brahma Vaivarta Purāṇa is one of the worst interpolated Purāṇas and I find it important to know which parts are interpolated exactly. The Kṛṣṇa Janma Khaṇḍa contains some shocking vulgar eroticism and even meat hangs out to dry in Nandagrām when Uddhava enters the village. Then there is the following 12 verses which are often quoted to prove predictions of a 10,000 year golden era in Kali, which is mentioned nowhere at all in any of the other ācāryas’ books. These are the verses. Do they (especially verse 59) really speak of a Golden Age? Are these verses genuine?
I cannot say which parts are interpolated but surely there are interpolations. My reasoning is that Śrī Jīva Gosvāmī quotes many verses from Brahma Vaivarta Purāṇa and they are not to be found in the available editions. Moreover, what is available now, he does not quote. If they had been available in the Brahma Vaivarta Purāṇa at that time, he would have definitely quoted them, because they would also have been useful to his work. Jīva Gosvāmī also mentions the story of Śukadeva’s birth, in which it is said that he did not want to come out of the womb, and took birth only on Krsna’s assurance that His māyā will not trouble him. This is supposed to be in Brahma Vaivarta Purāṇa but is not seen in the available editions. Some research should be done what is interpolated.
In Bhaviṣya Purāṇa there are also descriptions about Caitanya Mahāprabhu, Guru Nanak, Kabir, etc. It is over 20 years ago that I read Bhaviṣya Purāṇa, but if I am not wrong there is a story of a meeting between Mahāprabhu and Bhaṭṭaji Dīkṣit, the author of Siddhānta Kaumudī. Now Bhaṭṭajī lived even after Jīva Gosvāmī, so how could he have met Mahāprabhu? I remember when I read it, I lost all faith in these stories. There is also a verse in Bhaviṣya Purāṇa which says that in Kaliyuga ravi-vāsara (Sanskrit name for Sunday) will be called Sunday. That is quite a far out prediction. So definitely here are interpolations in Puranas. Some have more interpolations and some less.
I have not come across any prediction of 10000 years of a Golden Age in Kaliyuga in the Purāṇas.