Is it permissible for a driver with diabetes, who travels long distances and cannot find a place for ablution (wudu') and prayer (salat), to combine the Dhuhr and Asr prayers upon arriving home? And must he perform the supererogatory (sunnah) prayers after combining them, or should he abandon this practice?

The matters of purification and prayer during travel are made easy. You can carry water for purification, and if water is not available, then tayammum (definition) (dry ablution) and istijmar (purification with stones/tissue) are permissible. Wherever prayer overtakes you, pray; for the Prophet, peace and blessings be upon him, said: «The earth has been made for me a place of prostration and a means of purification. So, wherever a man from my Ummah is overtaken by prayer, he should pray.»

If the time for Dhuhr (noon prayer) enters while you are traveling, and you know that you will arrive before the time for Asr (afternoon prayer) exits, it is permissible for you to delay Dhuhr and combine it with Asr as a jama' ta'kheer (delayed combination), and you complete it as four rak'ahs.

The Sunnah (definition) (supererogatory) prayers before and after the obligatory prayers are recommended, not obligatory. Therefore, you are not required to pray them whether you are traveling or resident. If you wish to pray the Sunnah of Dhuhr when combining prayers, then pray the pre-Dhuhr Sunnah before combining Dhuhr and Asr, then the two obligatory prayers, and then the post-Asr Sunnah.