Must she make up the missed prayers and fasting, or offer expiations, given that she does not remember their number, and knowing that current family circumstances make it difficult to disclose this to her husband or to fast without his knowledge?

All praise is due to Allah who has accepted your repentance. We ask Him to grant us and you success and steadfastness.

Scholars have differed regarding making up prayers intentionally missed. The majority view, and the one we adopt for fatwa (definition), and which is the more cautious of the two opinions, is that they must be made up.

If you do not know the number of those prayers, then you should make an effort to estimate, and make up what brings you certainty or strong presumption that your obligation has been discharged. Then, you are obligated to make them up according to your capacity, in a way that does not harm your body or your livelihood.

Similarly, for the days you intentionally spoiled or broke your fast deliberately, you must make them up. If you do not know their number, then make up what brings you certainty or strong presumption that your obligation has been discharged. You do not owe expiation, because expiation is only due for breaking the fast through sexual intercourse, according to the majority of scholars.

You are not permitted to abandon making up these obligations out of fear that your husband will find out. For making up these obligations is incumbent upon you, and you must fulfill them. You are not required to inform him that this fasting is to make up for intentionally missed Ramadan (definition) fasts. Rather, you can give him the impression that this is a make-up fast for days you missed with an excuse, for there is room to avoid lying in allusions.

And Allah knows best.