侵入 and 入場: rain entering and admission to a venue
Compare rainwater entering through a window gap with entry available before a performance, reading the subject that enters and the timing and possibility of admission.
Rainwater entering through a window gap
侵入 (しんにゅう) describes intrusion or entry into a space. 「窓の隙間から雨水が侵入した。」 says that rainwater entered through a gap in the window.
雨水 is what entered, and 窓の隙間から identifies the gap it entered through. This example does not describe a person deliberately entering.
When entry to the venue is possible
入場 (にゅうじょう) means admission or entry to a venue. 「開演の三十分前から会場に入場できる。」 says that entry is possible beginning thirty minutes before the performance.
三十分前から marks the starting point for entry, and できる expresses possibility. The sentence reports neither completed entry nor a limit of only thirty minutes for admission.
What entered and the situation of admission
The 侵入 example reports rainwater entering through a window gap. The 入場 example gives the time when people can enter the venue for the performance.
Read what enters together with the situation. Do not restrict 侵入 to deliberate unlawful acts or expand the admission example into real ticket or permission procedures.
Does 「開演の三十分前から会場に入場できる。」 report that entry has already taken place?
Show the answer and why
No. It gives the possibility of entry and its starting point, thirty minutes before the performance.