Since Xiaomi provides a fully rooted rom i do not think it would viod the warranty. But that's just it unless we get something direct from xiaomi it's only opinions.....
as for ICS MIUI i do not think that would work.....
In theory you downgrade to ics but you need the Rom which is built for mi2 not just any build. The build you are talking is not for mi2 may be for mi1 and some other phones.