I think (although I have no proof or link ><) that it's almost sure the other band members have talked to Kyo about him hurting himself... because it is a problem for him... (I don't mean that in an insulting way to anyone.) I'm sure they would have tried their best to help him, even though it's not easy. In the end it's down to him and to cope with things in whatever way he can.
About smoking... it really does damage the voice. Just because you can't hear it straightaway doesn't mean it's not doing any damage. And of course, some people's throats are more resilient than others', so they could smoke and sing for years and maybe not notice anything, whereas other people could get affected more quickly.
In any case, a lot of smokers don't get problems with their throat or lungs until later in life, say, when they're in their late 40s, or their 50s and 60s. And I don't want anyone to get lung cancer or some such disease when they are old, neither!
Personally I think you
can hear the difference in Kyo's voice more recently, and I think the quality
is deteriorating (I mean slightly - I don't mean to say his voice suddenly became no good or something!). Although some of it might be to do with the hammer he gives his voice, rather than anything to do with smoking.
As for the other rumours - pfft! who will possibly know yet?!