1.1 million yen/year is the limit, Mikawa.
As for all of sumimasen's other questions, I would think that they would be rather personal answers. I mean, how are we to judge what is best for them and any future family matters? At least they are thinking of such things, but geez!
how to take care of finances if she has to stop working completely ?
Very carefully. As mentioned, she can take on part-time work if she likes (and is able. I would add one more caveat here. She is probably well aware that women around her age are not usually asked back to work once they leave it. Not for 20 years anyway (when kids are grown up, and yes, I know you don't have any yet). Think SERIOUSLY how her unemployment will be perceived by potential employers.
Also, depending on what she does for work, will her skill set be up to date (or improvable) in the future, should she decide to re-enter the work force?
how much additional amount of money i should spare aside to cover her monthly living expenses
To be honest, I really don't even understand this question. What do you mean by "her" living expenses? Is she going to spend more money by staying at home? With no kids around, what does she plan to spend money on? Ask
her for the answer you seek!
how to 'save' for kids in future and save for retirement
Life insurance plan, investments, additional savings plans (if offered from work), scrimping on things now so you won't have to later, etc. Like I wrote above, this is
very personal!