Absolutely, but it might not work exactly how you expected
Tremorsense can absolutely be used as you described. As tremorsense is defined (MM, p. 9, bold added)
...tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance. Tremorsense can't be used to detect flying or incorporeal creatures.
Tremorsense is usually used by burrowing creatures, enabling them to come up, attack their prey, and dive burrow back down. As such, the text above doesn't clearly define whether a creature can "see" a target through tremorsense alone. Your reading that "tremorsense lets you know where a foe is standing but doesn't let you see their body movements, so you are effectively attacking blindly" is completely viable.
Would attacks get disadvantage? Probably not
However, note that the rules for Unseen Attackers And Targets (PHB, p. 194-195) state both of the following:
When you attack a target that you can’t see, you have disadvantage on the attack roll.
When a creature can’t see you, you have advantage on attack rolls against it.
And even though your target can see the spiritual weapon, Jeremy Crawford has confirmed that the spellcaster is the one doing the attacking. So rules for unseen attackers apply here.
So if a druid Wild Shaped into an Earth elemental (having previously cast Spiritual Weapon), then dove into the earth via Earth Glide, and attacked a target they could pinpoint using tremorsense, the target would be "unseen" to them, and they "unseen" to the target.
And remember (PHB, p. 173):
If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20.
Thus, the roll would most likely be made normally, without advantage or disadvantage (this would not influence Flaming Sphere, since that spell does not involve an attack roll).
How powerful is this strategy?
This is a sound and clever use of tactics on the part of the Druid. But it's not insurmountable. The Druid can't cast spells while Wild Shaped (unless they are 18th Level or higher), so they can only maintain a spell, not renew it if needed. And as both of these spells are second level, a simple Dispell Magic spell will handily end them (unless they are upcast).
Keep in mind, a lot of tactics in DnD (or real life) rely on attacking while an enemy cannot counterattack. That's the whole idea behind most ranged builds. But that doesn't make these strategies all powerful. It will work, until it doesn't.