Can someone interrupt Salah if they fear danger?
Yes, as a matter of fact it becomes mandatory to do so. This comes out of the necessity rule in Islam. In Islam you can commit pronounced shirk, eat pork, drink wine, etc, let alone breaking your salah. This is clearly stated in the Quran when Allah says to:
Do not kill yourself as Allah is merciful on you [4:29]
Those who disbelieve after they believe except those who forced to pronounce disbelieve and his heart is satisfied with believe [16:106]
There are numerous evidence from Sunnah and Quran that always says your life tops EVERYTHING else. This also comes from the way Jihad used to be done where half would pray and half keep a watch for the enemy to warn those who are praying case they come.
Can someone interrupt Salah if they remember they can't lead anymore?
This includes reasons like getting the period, remembering that they don't have wudu or losing wudu during salah, etc..
This part there are two opinions by scholars, one is that it is allowed to substitute, aka
استخلاف Istiklaf. Those who said it was OK are companions and followers like:
- Omar Ibn Alkhatab
- Ali Bin Abi Talib
- Alhassan Albasri
- Ibrahim Alnakhi
- Abu Hanifa and all of his students
- Malik and all of his students
- Alshafai NEW madhab and all of his student
- Ahmed and the majority of his students
Which is the majority. Those who said it is not allowed, meaning that Salah needs to be repeated are:
- Alshafai Old madhab
- Some of the students of Ahmed in accordance to him mentioning it once to them
The majority take the opinion from the hadith, when Omar ibn Alkhatab was stabbed he stopped and pushed Abdulrahaman ibn Awf in his place to complete the prayer.