Is it permissible to shorten and combine prayers on the way back to our town when there are 30 kilometers left to reach it? And should we pray in a mosque that contains a grave?

As long as you have not reached your country, you are considered travelers, and it is permissible for you to combine and shorten prayers. If you know that you will arrive before the time expires for the prayer you performed on the road, you are not required to repeat it upon your arrival. However, if you know that you will reach the city before the time for the second prayer begins, then it is better for you not to combine prayers on the road. Praying in a mosque that contains a grave is forbidden by Islamic law. So, if the grave is inside the mosque, it is not permissible to perform prayers there. Performing congregational prayer is better for the traveler than shortening the prayer individually. And if you are a group of travelers, shortening the prayer with your group of travelers is better than performing it fully with the mosque's congregation.