Taxonomy of Asian Buteos is complex and consequently the identification of migrant/wintering populations in the region is much debated, best summed up in this paper http://www.caluta.liitin.net/Caluta/Caluta4.pdf Hopefully, 'jalid' will see this and comment further.
Just for reference, I tried to find among my pictures from buzzards in Kathmandu (Himalaya) the ones closest to yours and attach two. As well as a comb pic of recently seen buzzards and a juv Long-legged Buzzard.
The whole buzzard complex is so confusing that our two main bird guides (Grimett-Inskipp and Rasmussen/Anderton) recognise Himalayan for Nepal but not much more. Even the lines between Long-legged and Common/Himalayan have become more blurry. See pic of LL Buzzard juv. which is not unlike some juv Himalayans
I also attach a combo pic (video screengrabs) of three buzzards we saw two weeks ago, all in same area of Kathmandu valley. I resized them to the same wing span to show the stockiness of the first two (Common/Himalayan) and the longer wings of the third one, imo juv LL Buzzard (all opinions that can be contested)
As per eBird species maps, there are few eBird breeding season records of buzzards from the eastern (Sikkim/Darjeeling/Bhutan) and western Himalayas (Kashmir/Himachal), and none from the Nepal Himalayas. As birders hardly venture in the mountains during the breeding season/ monsoon, this might not be the last word about Nepal.
Arend in Kathmandu
The two pale birds really look like Long-legged (the bird in the third photo does look different individual to that in the fourth photo?). But note that long-looking wings are partly because juveniles of all species have narrower wings. As you wrote, the situation in the Himalaya is quite complicated. Upland Buzzard is also possible there (although these bird do not look like that species). The breeding status really is not well known. Also museum skins from the breeding season are sparse - like us nowadays, the collectors 100 year ago do not enter the mountains at the optimal season for taxonomical and breeding studies.
The bird in photo 1 is a typical adult Himalayan Buzzard, the bird in photo 2 is juvenile and less typical, but still belonging to the same taxon, I think. Then there is one adult of dark morph - that morph is restricted to mountains in japonicus group of buzzards.
Himalayan Buzzard and Steppe Buzzard are normally separable, and based on that, it seems that Himalayan Buzzard is scarce in the subcontinent away from the Himalaya, and Steppe Buzzard is scarce in the mountains.
Himalayan Buzzard and East Asian Buzzard (or whatever you like to call those buzzards of japonicus group, which breed in eastern Asia and spend their winter in S China and SE Asia) are very difficult to separate. I have tried to learn but only found out some average tendencies. Those average distinctions have not helped me to understand what is the relative status of Himalayan and East Asian Buzzards in SE Asia. The Thailand birds in the first posts really are adults belonging to one of those taxons. Some could argue that because Himalayan birds are not very migratory in Indian Subcontinent, they probably are not more to the east either and therefore the majority of buzzards in SE Asia should be of NE origin. I don't think that this is a valid argument, because the breeding distribution of Himalayan buzzards extends far to the north in Chinese mountains and it is almost certain that buzzards there are more migratory compared to the Himalaya.
Thanks Jalid, for your insights.
And yes, the fourth photo (juv LLB) is a different one from the ones in the combo photo.
I have no idea about how residential the Himalayan Buzzards of Nepal are. I only know that in Kathmandu the Buzzards (HB, LLB) appear normally mid October (one LLB in 13 years was seen here in September, 26th). The latest are normally seen end March, with very few seen ever in April. More detail I put in my book on southern Kathmandu valley birds: The Birds of the Bagmati and Taudaha Area, 2003-2016. There is a peak in November-early December when more than one can be seen per trip (day). Also the buzzards here might come from China rather than the Nepal Himalayas. Himalayan Buzzards arrive here the same time as Long-leggeds from farther away, while e.g. Booted and Bonelli's Eagle juvs (Himalaya breeders) can be also seen occasionally in Aug-Sep and May-Jun in our area (1350m asl)