All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
You are not Islamically obliged to pay your husband's debts, nor buy gifts for his parents, nor send a monthly salary to them, unless you do so out of your own will as an act of charity and kindness, and Allah loves the doers of good.
They are not entitled to exploit your wealth by forcing you to do what you are not obliged to do. The money that you earn is your own property and you are entitled to dispose of it in what is permissible as you wish. Moreover, your husband is obliged to spend on you even if you are rich; please refer to fatwas 193269 and 85541.
Our advice to you is to try to appease your parents-in-law and avoid their evil as much as possible.
The most important thing to achieve stability in a marriage is mutual understanding between the spouses and that they do not allow others to interfere in their lives, as such interference may ruin their relationship.
Hence, you should not have spoken to your father-in-law about this loan that your husband had taken as he is not Islamically obliged to consult you about it, but it would have probably been better if he had done so. The fact that he makes this matter a reason for divorce is not acceptable from him. Marriage is not an easy matter so that it should be weakened for the least of reasons. Allah, the Almighty, called the marriage bond a solemn covenant, as He says (what means): {And how could you take it while you have gone in unto each other and they have taken from you a solemn covenant?} [Quran 4:21]
Hence, we advise you to be patient and try to reach a mutual understanding, especially in regard to what you mentioned of planning and saving for the future.
Finally, we warn you against resorting to any un-Islamic transactions in earning money, as it is Islamically forbidden to borrow money with interest or to deal with commercial insurance and similar transactions.
Allah knows best.