Is it permissible to combine the Asr prayer with the Dhuhr prayer by performing them together earlier (jam' taqdim) due to illness and sleeping after the prayer until just before the Maghrib adhan?

If the fever and sore throat you experienced make it difficult to perform every prayer at its appointed time, then you are correct in combining Dhuhr with Asr, and your prayer is valid, and there is no blame upon you. Combining Dhuhr with Asr, and Maghrib with Isha, is permissible due to hardship and difficulty.