What is the ruling on reserving a place in the first row of the mosque by placing a personal prayer mat, and what are the implications of removing this prayer mat to pray in its place? And does one who arrives late attain the reward of the first row if he prays in it?

There is no harm in removing the rug placed by someone who reserved a spot in the mosque. Others are permitted to remove it according to the more evident of the two scholarly opinions. If the prayer time arrives and the person who reserved the spot has not shown up, it should be removed. And whoever comes late and prays in the first row will attain the virtue of praying in it. The praised first row is the one immediately behind the Imam, whether its occupant arrived early or late.