UTC just provides a labeling for TAI/TT seconds, so a count-of-UTC-seconds timestamp is identical to a count-of-TAI-seconds timestamp (assuming the same epoch). Both are different from a count-of-non-leap-UTC-seconds aka POSIX time.
You cannot know for sure how many leap seconds there will be in for instance the next 5 years. So you cannot know for sure exactly how many seconds there will elapse from now until for instance July 1st 2020 midnight UTC.
Yes, but we don't know what that labeling will be in the future.