情報の真偽を確かめるまで帰れなくなった
What does なくなった add? I'm parsing なくなった as なくなる or the negative form of なる to become. So loosely it sounds like, "you will lose the ability to leave"。
情報の真偽を確かめるまで帰れない
Is this the same or correct when just using ～ない, not allowed to leave？
The verb 「なる」, which is a key verb in the language, always describes a change of state as in:
・A turns into B.
・A becomes more/less (adjective).
・Situation A changes to situation B.
「情報{じょうほう}の真偽{しんぎ}を確{たし}かめるまで帰{かえ}れなくなった。」
This sentence talks about a (fairly recent) change of state as below:
Old state: One was free to leave or was about to leave.
Something happens that changes all that. ← Perhaps a new piece of information has come in.
Present state: One cannot leave until the authenticity of the information has been verified.
Old state （帰れた）⇒⇒ New state （帰れなくなった）
「帰れなくなった」＝「帰れる」+「ない」+「なる」 in the past tense
Thus your first sentence means something along the lines of:
"It has (just) become impossible for me/us to leave until the authenticity of the information has been verified."
Your second sentence:
「情報の真偽を確かめるまで帰れない。」
says nothing about a change of state (like your first sentence does). It just means:
"One cannot leave until the authenticity of the information has been verified."
