I think it’s probably safe to guess that the whole unit is covered, likely owing to the trend that affects that have a limitation will note it saying either models or units wholly within the area. It’s probably best to work this out with your opponent before hand just to avoid a disagreement later in the match
Generally speaking: A unit is in range of something if at least one of its models is in range. Abilities that only work when units are wholly in range say so. Abilities only affecting a few models of a unit are incredibly rare.
Rules in AoS default to the broadest possible case. So if there's no limitation like "wholly within" you get to use the broadest interpreation possible.
Pretty much. When it's not specified "wholly within" , you only need to have 1 model into range to affect the entire unit.