What is the ruling on praying Maghrib without purity due to ritual impurity (najāsa) that has afflicted one's clothes, and the inability to remove it or change them because the home is far away and no transportation is available? And what is incumbent upon someone who fears missing the prayer time and is unable to purify themselves for it?

The actual impurity must be removed by eliminating its substance, and the an-Najasah (definition) al-Hukmiyyah (impurity of حكم / ruling) by pouring water over it until it is submerged. If a Muslim is unable to remove or reduce the impurity, their prayer is valid, because the removal of impurity is conditional upon knowledge and ability. Whoever prays despite being unable to remove the impurity has fulfilled their obligation. And whoever fears the prayer time will expire and is unable to remove the impurity should pray according to their situation. As for one who is unable to perform Wudu' or Ghusl (definition), they should perform Tayammum (definition), as Allah Almighty says: "And if you find no water, then perform Tayammum with clean earth and wipe over your faces and hands."