First of all; it's good to see you back, ES.
I, for one, really missed you.
Arab incursion on India was to no further than Multan, though. If so than maybe Arab conquest to the east would be only up to Persia and Bactria (Afghanistan). I don't know if this will negate any more muslim incursion to the region, but I don't think Islam can't spread with other means, such as prosetylization through trade, like usual. And because the general Arab interest towards eastern goods, such as pepper, I don't think the possibility Indonesia and the other Malay states going to be muslim TTL wouldn't be likely. I mean even before the Umayyads seized the control, an Arab emissary had already visited China...
I'd say that an Islamization of Indonesia is going to be less likely without a strong Muslim presence in India.
Not impossible, but certainly less likely.
Indeed, there was a (very small) presence of Muslim traders in Indonesia from as early as the 8th century, but it was for a good part due to the rise of Muslim states like the Sultanate of Bengal that Muslim merciants became so influential in Indonesia in OTL.
Without a powerful and wealthy Muslim state in Bengal, the influence of Muslim traders is going to be weaker, as there was a lot of trade between Indonesia and eastern India (and specifically Bengal). And without a strong Muslim presence in Bengal, Muslims will play only a minor role in this trade, as opposed to the significant role they played in this trade in OTL.
And we shouldn't forget about the fairly significant role that Bengali Muslim missionairies and scholars played in the propagation of Islam in Indonesia.
Exactly what
would happen in Indonesia without a Muslim Bengal is hard to say, as the Islamization of Indonesia was a very complex process that took centuries and in which lots of different factors were involved.
But nonetheless, the OTL Islamization of Indonesia really got started when the Majapahit Empire began to crumble and a number of Muslim principalities (along with Hindu principalities) arose in its wake. These Muslim principalities, strenghtened by their ties with the rest of the Muslim world, then conquered or absorbed most of the remaining Hindu principalities, resulting in Islam becoming the dominant religion in the region.
Now then, if the influence of Islam and Muslim mercaints does not become strong enough to lead to the rise of significant Muslim states in the wake of the Majapahit's decline, then the Islamization of Indonesia could very well be butterflied away, with Islam not becoming any more significant in Indonesia than it became in, say, Kerala or Sri Lanka.
Further questions: Buddhism was still a powerful force in parts of India, Kashmir and Bengal. How would they manage without pressures from Muslim missionaries?
The political and religious influence of Buddhism in India was actually already deteriorating when the Muslims invaded.
In fact, by the time the Ghurids and Ghaznavids invaded (i.e. the first time the Muslims actually got a firm foothold in northern India), North Indian Buddhism was already in the process of being reabsorbed into Hinduism.
The decline of Indian Buddhism certainly was worsened by the destruction of many Buddhist temples, monasteries and universities at the hands of the Ghurids and Ghaznavids, but even if these invasions don't happen, Indian Buddhism is, in all likelyhood, still going to face a serious decline.
Buddhism in India could very well remain stronger than it was in IOTL, but unless it is reinvigorated by the rise of a new Buddhist empire in northern India (this could happen if the Mongols conquer northern India ITTL before becoming Islamized, and subsequently adopted Buddhism - many of the Khans of the Chagatai and Il-Khanates were Buddhists, so there could very well be a number of Buddhist Khans in an ATL Indian Khanate), it is still doomed to become a minor religion in India.
A lot depends on where you define India to be, but Buddhism may be stronger in the north (for a lot of the same reasons Islam took root among so much of the lower castes).
True, but Buddhism already was stronger in the north than in the south, and northern India also produced at least two great Buddhist empires; the Maurya empire and the Pala empire.
The south, on the other hand, remained overwhelmingly Hindu, while Buddhism IIRC never really gained a firm foothold there.
With the exception of Sri Lanka and IIRC the Maledives(*), of course.
* IIRC the population of the Maledives was Buddhist prior to its Islamization, but I'm not 100% sure on this.
Point of order, Islam was spread by trade an missionary work rather than conquest (even in India there was a lot of bottom-up conversions the local overlords did not activly seek).
Yes and no - indeed Islam was often spread by merciants and wandering Sufi mystics, and there were indeed quite a few of local Muslim rulers who were more interested in tax-farming or occasionally looting the non-Muslim population rather than make attempts at converting them.
But you should keep in mind that the presence of these merciants and Sufis was, either directly or indirectly, the result of political Muslim dominance in the area.
After the Ghurids and the Ghaznavids had consolidated their conquests in northern India, an influx of Central Asian Muslim merciants and Sufi preachers followed, particularly after the Mongols had conquered and destroyed the Khwarezmian Sultanate.
In other words; though these bottom-up conversions you mention were indeed often the result of merciants and preachers who were often acting independantly from the local Muslim authorities, their presence in India was nonetheless still a result of the Muslim conquest.
Without the Muslim conquest of India, Muslim merciants would have far less influence in India, and the economic ties with Central Asia wouldn't be quite as strong and close as they were in OTL.
And without the Sultanate of Delhi as a refuge, most of the refugees from Khwarezmia will most propably end up elsewhere, such as southern Persia or the Caliphate of Baghdad.
Thus, there is still going to be significantly less Muslim influence in northern India without the Muslim invasions.
Islam may have less of a presence on the subcontinent, but would be more heavily weighted toward the south.
I agree - southern India would still be exposed to Arab, Persian and Swahili Muslim merciants, and a lot of them would end up settling there, thus establishing a fairly small but still significant Muslim community.