Is it permissible for one to masturbate in order not to commit fornication?

This is called committing the milder of the two evils such as eating a carcass in fear of death, and this happens when committing fornication is very likely to occur.

I`m infatuated with a man, is it permissible for me to make supplication that he falls in love with me and becomes my husband?

Islam has honored women by being proposed to by men, and not the other way around, so it is inappropriate for a woman to propose to a man since one who hastens in asking for a thing prematurely shall be punished by deprivation. In fact, such a phenomenon is the result of unlawful mixing between the two sexes.

A man donated a burial plot, but before registering it in favor of the Ministry of Awqaf and before burying anybody there he said that he didn`t want to donate it and planted it. What is the ruling of Sharia on this?

All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

If he had endowed it as a cemetery, then he can`t take it back, even if he didn`t register it in favor of the Ministry of Awqaf. If he said: "I have endowed this plot as a cemetery, then it becomes an endowment." However, if he didn`t endow that plot-but it was his intention to do so-then he is allowed to back down. And Allah The Almighty Knows Best.

Is it permissible for a woman in her waiting period (Iddah) after her husband's death to leave her house and mix with non-Mahrams?

It is not permissible for a woman in her waiting period (Iddah) after the death of her husband to leave her house except for a necessity, such as for medical treatment, nor is she allowed to mix with non-Mahrams. A woman is not permitted to mix with anyone other than her husband or Mahrams, whether she is in her waiting period or not. However, there is no harm in accepting condolences from those who visit and responding to their inquiries about her well-being. And Allah Knows Best.