To sum up your questions in general, playing D&D for a long time and experimenting with the game, how adventurers would react, what 'role' a half dragon would play in the scheme of things is up to you. Have them play a vital role of a villain in a module, PC's can try and reason with them, it all just depends how you and your group want to run things.
The origin of half dragons are commonly amongst evil societies, red dragons shape changing into humanoid form before going to bed with someone, you get the idea. Statistics on them, I don't know where to get them because I don't think they exist in 2nd or 3rd edition D&D.
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