Nama is independent of diksa, but nonetheless he chooses to reveal himself through diksa. Therefore to disregard the guru (and think one does not need to take diksa) is one of the ten nama aparadhas.
The argument that siksa is more essential in no way dismisses the need for diksa. But the seed of diksa without the water of siksa (sambandha jnana) will not fructify.
Attaining spiritual perfection is very rare, even mukti, what to speak of aisvarya-mayi prema and more so madhurya-mayi prema. With no diksa and no siksa, it would surely take more than one life. So in the next life (arguably) the chanter would find the guru (or his representative) who put the maha-mantra message in the bottle, his guru.
The Vaisnava is the most prominent manifestation of Krsna’s krpa-sakti, potency of mercy.
Accepting diksa is not essential in every case. This point is raised by Jiva Goswami in his Bhakti-sandarbha. However, he explains that it may not essential for those are already sense controlled, etc. However, this is not in reference to raganuga bhakti. In raganuga bhakti the guru becomes more important as one advances. We see in almost all cases that the disciple follows the bhava of his or her guru. First the guru is seen as the representative of God, and then as one progresses, as representing a particular ananda-krsnanandaya dhimahi—that the disciple follows into the lila. Exceptions can be explained in a number of ways.
A siksa guru can serve in the capacity of raga marga guru. Mahaprabhu’s raga marg guru was Ramananda Roy.