If I don't have any characters with the same number of stars or fewer than my opponent's character, can I still reveal Lose the Initiative to stop their character from untapping?
[Source: WotC]
Yes. Obviously, you won't untap any of your characters, but the original effect of your opponent untapping their character will be replaced with nothing happening.
If a combiner combined and untapped. Would Lose the Initiative cancel the untap combiner effect?
[Source: WotC]
A team of characters combining isn't actually untapping; the resulting
Combiner character is entering the battlefield untapped (like all characters which enter the battlefield after the start of the game, unless another card or ability says otherwise).