It depends on the spell.
Here's how Tremorsense works, according to the Monster Manual:
A monster with 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.
Notice that Tremorsense doesn't give you to the ability to see. It lets you know accurately where vibrations are coming from, when things move or make a sound against material that you're in contact with.
Since it doesn't let you see, you would not be able to use a spell like Call Lightning while fully embedded in the earth. The spell requires you to "choose a point you can see within range", and you cannot see anything.
But spells like Flaming Sphere or Spiritual Weapon would work just fine, since they don't have that requirement. Flaming Sphere says that "you can move the sphere up to 30 feet" as a bonus action. Spiritual Weapon has similar wording. Since these spells don't require you to be able to see, you should be able to use them while relying purely on Tremorsense.
However, the enemy may figure out that they can just stand perfectly still to avoid giving away their location, or distract you by throwing things onto the ground.
Note that you make attacks with Spiritual Weapon at disadvantage if you're relying purely on Tremorsense. According to the Player's Handbook ("Unseen Attackers and Targets"):
When you attack a target that you can't see, you have disadvantage on the attack roll.
Geas itself doesn't say.
In principle the victim just has to understand the orders at the time they're given. However, if they don't remember being given orders, they may have no idea of what they have to do to satisfy the geas. That's a bad situation to be in.
One way to do what you describe would be for the NPC to have cast geas earlier, but not given any instructions yet. Then instead of speaking a trigger word, the traitor would just say "Protect me" and the geas would enforce the command.
Geas is not a great tool for this.
The problem is that the traitor needs the victim to cooperate for the duration of the fight, but geas will only zap them once per day. If the player decides to disobey, he will instantly take 5d10 damage, without warning. If he survives that, he's now completely free to disobey again, as much as he wants, with no fear of reprisal.
For this purpose I'd consider homebrewing a geas variant that lets the caster choose how much damage to inflict for each act of defiance, up to some cumulative maximum. This lets them fire some warning shots.
Discuss this with the player, privately.
One failure mode in this whole plan is that many players resent having this stuff sprung on them. Because they weren't prepared to betray the real people sitting at the table with them, they'll likely "fight" on behalf of the traitor as ineffectually as they can. In practice this means either that player sits on the sidelines during a protracted fight against the traitor, or the traitor gets curbstomped by the rest of the party, depending how tough you make the guy.
(Another issue is that geas applies a charm effect and the player needs to know they're charmed so they can act accordingly. Not telling them in advance greatly increases the risk that you'll have to tell them at the table and tip off the rest of the group.)
To have a PC with unreliable memories, you need some level of buy-in from the player. This doesn't have to mean giving away the twist, so long as the player knows there is a twist and is willing to go where it leads. You can say something like "I'm thinking that while you were away from the party, some things happened to your character that they don't remember, for reasons that will become clear later. Are you OK with that?"
Example. I had a PC who barely escaped an attack by werewolves, and got infected with lycanthropy, but didn't know it. I texted the player and said "Hey, your character is now a werewolf. In 2-3 sessions we're going to have a full moon, and you'll turn into a murderbeast." He thought this was pretty awesome, and when the time came he competely ran with it.
Best Answer
You can't use modify memory, but if you found another way...
From MikeQ's answer, a revenant is immune to the charmed condition, which makes it immune to the spell modify memory. But if you were to find some other way to alter a revenant's memory, one which bypasses its immunity to the charmed condition (wish is the obvious answer, but there are better wishes the BBEG can make so you should probably devise a more original solution. Maybe some ability which lets the BBEG ignore the charmed immunity on undead, if he's a necromancer), then there still remains the question of whether an altered memory will change the revenant's sworn targets.
The description of the revenant in the Monster Manual (p.259) says that
However, it says nothing about magic's ability to make the revenant pursue a different creature. On one hand, one could argue that a revenant is fueled by a soul's desire for revenge, regardless of whether it is correct or not, and that it only matters who the revenant believes has wronged it, regardless of whether the target is actually guilty. This could apply if revenants emerged from some process independent of external agents. This view is supported by the first paragraph of the revenant's description, which describes the soul acting by its own agency. If this is the case, then altering memories could alter a revenant's target.
On the other hand, it is possible that a revenant's target is fixed from the moment it is raised into undeath. The following two paragraphs of the revenant's description, Hunger for Revenge and Divine Justice, hint at this powerful supernatural link between the revenant and its target. If revenants are raised by a god so it can enact divine justice, then no mortal effort could alter the target. If this is the case, then altering memories will not alter a revenant's target, but it might leave the revenant rather confused.
So either case can be argued to be correct. Whichever way you rule, you should be consistent. If the PCs learn that revenants can have their target changed via whatever means the BBEG used, they might use this tactic later if it uses abilities and items which PCs acquire, although since NPCs are not PCs the BBEG could very likely be using some ability which is beyond what is available to PCs.
But since there is no rules-as-written answer to the revenant's behaviour (at least, not that I can glean from the Monster Manual), the real question becomes 'which option makes for a better story?' In my opinion, having the BBEG trick a revenant into hunting the party by using memory modification is a cool story and you should do it. Note also that there are multiple possible outcomes to this plot. Beyond combat, the player characters might be able to detect and reverse the curse which was placed upon the revenant, at which point they would gain a useful ally and strong lead towards the BBEG. This sounds like a brilliant plot arc and you should do it. Even if the official revenant description opposed this plan, you could overrule it to make for a better story.