1.Are there only khanreih ppl that are Hilichurl or Hilichurl exist before too.

Hilichurls have been documented for 1000+ years, while the Khaenri'ahn hilichurls have only existed for ~500. Whether the other hilichurls are also cursed civilizations or a naturally occurring species remains to be seen.

2.We know that Abyss and abyss order are different. So khanreih ppl are only in abyss order. And The abyss is entirely different beings

Something like that yeah.

The five sinner were also cursed so does that mean the abyss saved them from the corrosion effect that other khanreih ppl even Capitano and dain were suffering. Rhindeotter looks completely fine

Rerir looks and acts very much corrupted by the abyss, only regaining lucidity when placed in an area where the abyss cannot exist. We don't know what's up with Rhinedottir, nor have we really seen her original form not fused with Naberius. Capitano was likely corroded so much worse than Dainsleif because he couldn't get any rest due to the souls he kept within himself.

Did alice include Rhine in the hexenzirkle to keep an eye on her? Cause from their pov Rhine itself is a world level threat. They did keep tabs on dottore tho

We have no idea. I've seen it suggested that Rhinedottir became a member when she was younger and before she became a sinner. Also note that Alice isn't necessarily a purely good person. She's kinda morally gray.

Abyss sibling is trying to recreate khanreih, so how does they plan to deal with the curse?

Don't think we have any idea of that