As you point out, the [*pass without trace*][1] spell description is somewhat ambiguously phrased regarding both points. In addition, the [Sage Advice Compendium][2] does not address such edge-cases regarding the spell. 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][3]:

> *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][4].)

#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][5]:

> *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.