I don't think it's altogether a bad idea, it depends on how you implement it.
One of the ways it can be achieved is to have the present day story and the past story running in parallel. This would mean that events would need to develop for the character in prison, whilst he remembers back to what happened previously.
Answering your question about the scene, having it not plot-oriented or action-packed is very useful, as it will be a time in the character's life where all that he can do is sit and think, therefore he would naturally reminisce about the events that led him there.
The past story would need to develop faster than present one, in order for the timelines to catch up, but it would allow intermissions from the memories to progress the current story slightly. This may simply be the character talking to one of the prison guards about something (like how long is he going to be stuck there), then going back to remembering.
The present story would likely need to eventually progress significantly, such as the character breaking out of the prison within the first book, but without the past story having caught up yet. This way when the past story reaches the part where he is imprisoned, the reader will understand that the two timelines have merged and the story can then be simply told from the present setting.
This way, rather than catching up to the current story in the middle of book two and continuing from there, the end of book two will culminate at the point that you have originally planned, as well as simultaneously allowing them to see how the main character originally landed in jail.
To answer your other question, this is a done thing: a similar example of this is in the book The Name of The Wind by Patrick Rothfuss, who has a character recount tales from his past whilst living in the present.
However, this style has limitations. Firstly, the story would have to be told entirely from the main character's point of view. Otherwise it will make no sense having multiple POV character's when the story is about the experiences of a single person in the past.
In addition, it will need to be made very clear whether the part of the story currently being read is in the past or the present. The example I gave switches from 3rd to 1st person, making it obvious what part of the story is being developed.
You say you're writing a series: I would say it wouldn't make much sense to write in this style to plan on merging story-lines in book 2 if you're planning on doing 7 books. It would change the style of book too much over the course of time, meaning readers might invest in the story but then decide they don't like the change in writing style later in the series.
I would say three books would be acceptable, with the last book the culmination of the character development from the first two books (the character likely learning in the present day from his experiences in his past).
There are probably other ways to do this type of story, but hopefully this will give you at least one idea.