一時 and 一斉: temporary states and simultaneous group actions
Compare pausing music with three machines stopping together, distinguishing the duration of a stop from the group involved.
A temporary state
一時停止, read いちじていし, means a pause. 音楽を一時停止しました。 says that the music was paused.
一時的な変更です。 says that the change is temporary. It does not fix the exact time at which the previous state will return.
Several subjects acting together
一斉, read いっせい, indicates a group acting all at once. 参加者が一斉に立ちました。 says that the participants stood up together.
三台の機械が一斉に止まりました。 says that three machines stopped together. No duration for the stop is specified.
Duration, the group, and clock-time wording
三台の機械を一時的に一斉停止しました。 says that three machines were temporarily stopped together. 一時的に concerns the temporary state; 一斉 concerns the shared timing.
In 一時に集まります。, 一時 can be read いちじ to specify meeting at one o’clock. Unlike 一時停止, this is a clock-time context.
Does 三台の機械が一斉に止まりました。 establish that the machines stopped permanently?
Show the answer and why
No. It says that three machines stopped together. The duration of their stopped state is not supplied.