[RPG] attack with the Spiritual Weapon spell as an action, and how to determine the target

A spiritual weapon (created by the spell of the same name) uses your bonus action for movement and attacking. It does not have its own actions, bonus actions or reactions.

The Spiritual Weapon spell has a casting time of 1 bonus action and range of 60 feet. Breaking down the spell description:

You create a floating, spectral weapon within range ...

So, you use your cleric's bonus action to cast the spell, creating the spiritual weapon within 60 feet of your cleric.

When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon.

If a creature is within 5 feet of where you created the spiritual weapon, as part of the same bonus action you used to cast the spell, you can attack that creature with the spiritual weapon.

As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it.

In every round after you cast the spell, for its duration, you can use your bonus action to move the spiritual weapon up to 20 feet and attack a creature within 5 feet of the spiritual weapon.

Since any use of the spiritual weapon spends your bonus action, your cleric still has a regular action (and reaction) available.

It's a DM ruling, but I'd say no.

As you show, nothing in the text of the spell discusses whether it's a valid helper for sneak attack. Therefore, we have to look at other factors.

The Sneak Attack text says,

you know how to strike subtly and exploit a foe's distraction.

But is a spiritual weapon distracting? Note that it doesn't do anything unless ordered by its caster. In particular, it can't make opportunity attacks. Consider the text for opportunity attacks:

In a fight, everyone is constantly watching for enemies to drop their guard. You can rarely move heedlessly past your foes without putting yourself in danger; doing so provokes an opportunity attack.

However, you can move heedlessly past a spiritual weapon, because it's not on guard and looking for an advantage. In the absence of any other text, it's simply floating there until commanded. Thus, on your turn, it's not posing an immediate threat the way another creature would.

Additionally, Sage Advice says no, though without an explanation.