Regarding your actual questions which are very important and well thought out, Shuayb has given an interesting reply which distinguish the crude polytheism of the pagan Arabs from the 'polytheism' of Hindus. For my part, I would say the following: Scholars have taken different opinions regarding the 'status' of the idol worshippers of the jahiliyaa period prior to and during the coming of the Islam of Muhammad (saw). Some do say, as you stated, that their deviations stem originally from a revealed religion. Others state that the verse in the Quran that "li qulli Ummatin Rasul" ("To every Ummah We have sent a Messenger") was fulfilled for the Arabs through Sayiduna Rasoolulllah (saw), with the udnerstanding that the Prophet (saw) was also sent to all of mankind. This would mean that the Arabs did not infact have a Messenger--and thereby an authentic Message--come to them until the Prophet (saw) was sent to them. The remants of the din al-hanif and the hunafa that were very few in number (most of whom accepted Islam at the time of Muhammad (saw) who himself was a hanif and never an idol-worshipper) were seen as a marginal phenomenon and offshoots of the message that Sayiduna Ibrahim (as) brought. As such, according to these scholars the hunafa were not at all organically related to the idol-worshipping Arab traditions.
Even so, when a Rasool is sent 'in person' to a community, this in effect is arguably the only time and place when there is a 'black and white' distinction between Iman and Kufr as the light and guidance of a prophet is a witness for or against the darkness of one's own soul at the very moment of revelation: this is also why the Quran refers to kafirun (disbelievers, rejecting God, atheists), munafiqun (hypocrites) and mutaqqin (those who are God-concious). However, as time passes and the Messenger leaves this earthly realm, the dichotomy of iman and kufr arguably becomes more shaded and more grey, form that moment on till the end of time. This is one way to understand why the hadith regarding the denying the 'proper hearing' of the Messenger and what he (saw) came with, receives a nuanced and conditional understanding when it deals with non-Muslims who did not 'see' the Prophet (saw) and lived in another time and place and only 'heard' of him later. The two categories are not equal and so the potential for reward/punishment is not equal either. Had the Prophet Muhammad (saw) come to the 'Hindus' in their land in person (saw), then I think that would have been a much different situation. But he (saw) was sent to the Arab pagans who arguably never had an authentic wahy come to them before.....
The idea of 'dilution' that you mentioned applies more to those people who were considered to be 'ahl al-Kitaab' People of Revealed Scripture. As you know, the Quran explicitly identifies the Jews and Christians are People of the Book. As such, along with apprasing their traditions and the divine origin of their respective traditions, and granting salvation--in principle--to them, it is these people that the Qur'an criticises for having distorted some parts of their texts (tahrif) etc. It is the result of such distortion that many Muslims believe is the cause for the difference (read deviation) of their beliefs such as the belief in the trinity, etc. As such, 'pure idol worship' has always been considered a man-made concoction as opposed to authneic revelation which by Islamic definition were sent with tawhid, although the followers of these religions may have deviated from this, etc....The polytheists in Mecca actually believed that Allah was a God along with the other 300 or so God, or that these 300 or so Gods constituted Allah. It is this man-made 'belief' that earned them perdition if they chose to remain in these beliefs acter a Messenger (Rasoolullah saw) was sent to them.
The case of Hinduism is much more nuanced and has always been debated by the scholars and there is ikhtilaaf on this issue, espeically amongst the Ulema of India who are in the best position to give a legal opinion on the matter. These scholars ranged in their opinions from this Kashmiri scholar (Imam Khaffaji whose fatwa is the start of this reply) who condemns any one who takes any aspect of the beleif of others as 'true' apart from Islam as pure kufr, to scholars both in Kashmir and Hindustan who have included Hindus in the category of the ahl al-Kitaab since these people were found to have scriptures and since teachings such as Vedanta for Brahmins which stem from the Vedas etc, display a very pure expression of 'Tawhid' (which in their religious language is called 'non-dualism', which is another way of saying Divine Unity). For these reasons there has not been abolutely one position on Hinduism from a Muslim perspective even amongst Muslim Indian Scholars. It is from within this flexible framework regarding the religious other that contemporary Muslim scholars who uphold perennialist theses such Seyyed Hossein Nasr present their nuanced positions on Hinduism and other religions. As such, their perspective should at the least be respected and not condemned as pure kufr, even if we disagree with them, or even if they are 'wrong'....
To the following question, Nasr replied in the following during an interview that can be found in a bilingual Arabic/English journal titled "Religions/Adyan". The second paragraph of his answer I think is quite pertinent to what you have asked.
"When considering the disconcerting diversity of religious faiths among religions that range from monotheism to non-theistic and polytheistic, what can we see as common grounds?"
It should be noted that Nasr is expounding 'one Muslim way'--grounded in evident Islamic percedent-- of looking at the question of 'common grounds' between the religions in general, and Hinduism in particular. I have already outlined that there is legitimate range of ikhtilaaf on such issues in Islamic history and this above position need not be accepted as true by any Muslim. That being said, I feel there is a lot of insight in this point made by Nasr, especially in our context (islah).
In regards to your specific point:
"My main issue is the laxity in your condemnation against these beliefs that puts perennialist philosophy in a pickle from my perspective since Allah

and Muhammad

weren't as lenient either"
I have addressed some of this point above already. However, Muslims such as Nasr do not deny Islamic exclusivity. Notice that Nasr stated the following in the above reference: "However, polytheism in the Hindu sense must not be confused with the latter form of polytheism. Hinduism is based on the manifestation of one single Divine Principle in multifarious forms, which we in Islam do not accept in physical form...".
In this light, from the many posts of Shuayb and NurMuhammad12 that I have read on this forum, one can see that they do not explicitly deny Islamic exclusivity either--they understand it differently than normative Muslims do however. They accept Islamic exclusivity and deem it necessary on one level of religious significance, or otherwise the Surah Kafirun would have no meaning for them as Muslims, and as such, they would not be Muslims since they would be rejecting what is clearly in the Quran. However, with those who either are explicitly labled "people of revealed Scripture" or those whom Muslim scholars encountered as Islam expanded to include lands other than Arabia, and then recognized such peoples to be 'people of scripture' such as Zoroastrians and Hindus, Islam 'excludes' them all by defining Islamic orthodoxy in recognizing the Messengership of the final Prophet Muhammad (saw) as binding, but Islam 'includes' them as well, as those to whom an authentic revelation was sent. As such, they are recognized, their differences accepted so long as those differences are not explicitly criticized by Allah (swt) in the Quran, and they are not castigated to the hell-fire in principle, only if 'in fact' after 'properly hearing' of the Prophet (saw) and that he has come to perfect their deen, etc, they 'actively reject' him (saw) and do not return to tawhid, or do not heed the final call of tawhid in the form of the Islam of Muhammad (saw). One can recognize how this category of the 'categorically damned' is extremely nuanced and ultimately can only be known by Allah (swt). I believe this nuance gives perennialist Muslims a lot of flexibility and lee-way to expound a 'perennial philosophy' which is in consanance with Islamic principles--regardless of what certain Muslims may wish to believe.
Allahu A'lam.
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