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As you point out, the pass without trace spell description is somewhat ambiguously phrased regarding both points. However, rules designer Jeremy Crawford has unofficially clarified both cases on Twitter.

It benefits only those who stay within 30 feet of the caster

Crawford clarified whether the benefit lasts even if those targeted move out of range in a January 2016 tweet:

Targets of Pass w/o Trace must remain within 30 ft. of caster in order to maintain benefit?

Pass without trace is meant to benefit you only while you're within the spell's radius.

This seems like a reasonable interpretation of the spell's description, since the way it's said to work is that "A veil of shadows and silence radiates from you". It only extends a certain distance from the caster. (How the Stealth check is handled once a target leaves the 30-foot radius is ambiguous, but I think the simplest and most reasonable way to handle it is simply to subtract the added +10 bonus from their previous Stealth check - since otherwise your Stealth check is kept until you're discovered or you stop hiding.)

You can change who benefits from it throughout the duration

Crawford addressed a question about the spell's targeting throughout the duration in a July 2016 tweet:

pass without trace: can different targets be selected each round for the duration or are only the initial targets afffected?

Pass without Trace: you can choose different targets throughout the duration.

Again, this seems to be a logical possible interpretation of the spell description: "For the duration, each creature you choose within 30 feet of you (including you) [...]". One way to interpret this is as Crawford has - that it's not a one-time choice, but a repeated choice you can continue to make/change throughout the duration.