Bequeathing building a Masjid that was not built until the bequeather’s death

Q: My mother (may Allah be merciful to her) died on 06/05/1409 A. H. In `Asir Hospital. She had given me a sum of thirteen thousand riyals before she died. She had requested me to build a Masjid (mosque) with that money on her behalf and she had mentioned that if the Masjid costs more, she would give me the extra cost. Is it thus permissible to take some money out of the rest of the inheritance with which this Masjid can be completed or should I myself pay for this extra cost without taking anything from my mother's inheritance?My mother left four married sons including myself, four married daughters, furniture, some tools, a herd of twenty sheep, and a sum of five thousand riyals or less. Moreover, debts of about four thousand riyals were owed to my mother, she was not indebted, and she did not bequeath anything. Besides, one of her sons owes her a sum of eight thousand riyals and another owes her a sum of two thousand riyals.I am the elder son and I would like to be good to my mother and do something that she will be availed of its reward such as voluntary Sawm (Fast), voluntary Salah (Prayer), or a Sadaqah (voluntary charity). Please tell me what are the charitable ways of disposition in which their reward can reach my mother? Also, is it permissible for me to make Du`a' (supplication) for my deceased mother, my father, my grandparents, and all Muslims that Allah may forgive them and be merciful to them or any other Du`a' of (Part No. 16; Page No. 439) goodness?On the other hand, is it permissible that we collect all my mother's inheritance including all the debts owed to her, the sheep, along with the furniture and divide this amongst her children and her husband (our father) before we build the Masjid? Or do we have to collect the inheritance, build the Masjid, and then distribute what remains amongst the inheritors? Should we request from her children to pay the debts they owe her or should we waive this? In addition, what should we do if one of my siblings refuses to receive their portions of my mother's inheritance bearing in mind that some of them declared that they will do so.