• sūtaḥ — a driver
  Nominative of sūta. The Mahābhārata is itself called Sūta-saṁhitā in some traditions, because reciters are sūta-class. The Bhāgavatam too is told by Sūta to the sages. The word carries narrative-authority as well as charioteer-duty.
• rathinam — the master of the chariot
  Accusative of rathin. In Kaṭha Upaniṣad 1.3.3 ātmā is the rathin and buddhi the sārathi. Here both are persons, but Gauḍīya reading retains the Upaniṣadic echo: the sūta must guard the rathin's ātma-presence on the field.
• rakṣet — should protect
  Optative of √rakṣ. The dharma-imperative. Rakṣaṇa is also the Lord's own promise (Gītā 9.22, yoga-kṣemaṁ vahāmy aham). Rakṣa-dharma flows through every relation in the parikara, downward and upward.
• āyuḥ-man — O long-lived one
  Contiguous join (āyuḥ + man) from synonyms. Vocative — 'one possessed of long life.' A respectful address by a junior. The driver's deference is structurally proper even as he disagrees.
• vibho — O my Lord
  Vocative of vibhu — the all-pervading one. The driver addresses Pradyumna as vibhu — recognition that Kṛṣṇa's son carries Bhagavān-status. Even when rebuked, the devotee never lowers his honorifics.
• dharmam vijānatā — by one who properly understood dharma
  Contiguous join (dharmam + vijānatā). Present participle of vi + √jñā — to discern. Dharma-vijñāna, not mere dharma-jñāna, is the practical knowing that acts. The driver claims this; Pradyumna will eventually accept the claim.

• Where could mutual protection rather than one-way service describe a relationship better?
• Whom do I both protect and depend on?

Like the wise steward of a household who explains, calmly, that he locked the door precisely because the master had fallen asleep — the driver answers without defensiveness. Rathī and sūta have a mutual rakṣaṇa-dharma; he has merely followed half of it.