All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible for a praying person to recite two different chapters after having recited Al-Faatihah (i.e. the opening chapter of the Quran); for more benefit, please refer to Fataawa 91267 and 92614.
As regards saying Aameen for the supplication of the Christian mother, it is permissible for you to say Aameen when your mother supplicates good for you, however, if there is transgression in her supplication, like calling upon 'Eesaa (Jesus) and the like, then it is not permissible for you to say Aameen for her supplication, because saying so, is confirming the transgression which is included in the supplication.
On the other hand, we urge you to be keen on calling her to Islam as Allaah may guide her and save her from Hellfire thanks to you. Your mother is the person whom you should most be keen in saving from Hellfire, therefore, you should call her in a soft manner and with wisdom in a way that suits her status as a mother, and we ask Allaah to open her heart to Islam.
With regard to the money which you owe to people, then in principle the rights should be given back to their owners, and one should not be negligent in this. Hence, you should endeavour to give back your loan to its owner, and if you are unable to do so, if the person to whom you owe the money is deceased for example, then you should give the money to his heirs. If you do not know who his heirs are, or that it is not possible to reach the owner or his heirs, then you may give the amount of money in charity on behalf of its owner, so that the reward of the charity will be for him, Allaah willing. If the owner of the money comes back later on, and if he approves the donation, then you are acquitted, but in case he does not agree, you are still obliged to give him his right and the reward of the donation will be for you.
For more benefit, please refer to Fatwa 89762.