Is Sinus node conduction necessary for heart beating?
The SA node is not necessary for contraction of the heart.

The reason that this works is exactly what you mentioned: schematics.

They are in sequence, but within that sequence, there are multiple autonomous signal generators in the heart, where spontaneous depolarization occurs to initiate contraction: the SA node, the AV node, and the purkinje fibers.

The SA node fires the most rapidly, 60-90 times per minute - which is the average normal heartrate, because in most people the signal for contraction arises from the SA.

The AV node fires at 46-60 times per minute.  If it receives the signal from the SA node then it depolarizes in sequence like the normal sequence we learn.  But if there is no SA node signal received, the AV node will depolarize on its own.

The last "generator" are the perkinje fibers.  If there is no signal from previous nodes, they will spontaneously depolarize.  However, they depolarize VERY slowly, at a rate usually under 40 bpm.  This severe bradycardia is potentially life-threatening, but allows the heart to keep some circulation going if there is a sudden complete AV node block - but prompt treatment is critical.

Regarding the lack of necessity of the SA node: 

In fact, in some cases of supraventricular tachycardia (for example IST, see here) that are not responsive to conservative treatment, they will actually ablate (surgically destroy) the SA node in order to slow the heartrate down because a persistently extremely high heart rate can be dangerous.

Good question!