Learn English – “It must have been .. ” or “It must was “

I think the primary source of your confusion is conflating three very similar uses of these words. “Break” and “down” can be combined in different ways in different grammatical functions to say different things.

“Breakdown” as one word is always a noun, specifically a catastrophic event that a car can have.

breakdown    noun

a failure of a machine to function : an occurrence in which a machine (such as a car) stops working
_{Source: Merriam-Webster definition of “breakdown”}

• My car had a breakdown.

“Break down” as two words is a phrasal verb, the first word of which can be conjugated to indicate the timeframe of when a breakdown occurred, e.g.:

• My car broke down near the circle around eight o’clock this morning.
• My car has broken down.

“Broken down” can also be an adjectival phrase that describes a car that has broken down, e.g.:

• My car was broken down until ten o’clock this morning.
• My car is broken down.

The way the verb to hold (hostage) is used in this sentence, is a form that already implies a long-term action, and as such does not require a continuous form to indicate this. Compare to 'The couple had lived in their apartment for eight years'; while this was obviously an ongoing action, the continuous is not required as to live already implies a long term!

Although you could say 'They had been living in their apartment for eight years', the same can sound odd to native English speakers when applied to 'had been holding', it sounds like you are referring to the short-term version of holding. To many people, it will sound like they were restraining her all the time; it paints a picture of a bunch of men holding on to this girl for four consecutive days.