Iran doesn't want Russia there - maybe that will change if their backs are truly to the wall but they are opposed to any potential occupation of the country whether that is from the US or potentially Russia. Yeah all their threats have been directed at wiping Israel off the map. Realistically they only have one shot and I believe though can't be certain that they know that so if they ever did use a weapon of mass destruction it would be against Israel. I don't really have enough insight into the mentality of those running Iran to know the likelihood of using a nuke given the capacity to do so but I know that isn't something supported by most every day Iranians. (EDIT: Actually not even sure there - all the Iranians I've met have had very similar views against the leadership of both Iran and the US and a desire to see peace in the middle east but that is probably part of the reason I've met them not in Iran).