Thinking out loud here (so to speak
). Gwyllion and Mellonin are pawns in Tharonwe's control. As such, Gwyllion is unimportant, and he has little time to spare for her dreams. He started by giving her a potion to get her asleep in the first place, but once she was asleep, Thraonwe has been using his will and that alone to keep her asleep. It is not yet to his purpose to waken, for he does not want Aeron and company to become too hopeful. So as to symptoms of Gwyllion, she will begin to sleep more peacefully, but under duress, being asleep at the will of another. That will be a cause of stress on her person, so there can be symptoms of illness not unlike Frodo's from Weathertop to Rivendell, but nothing nearly so serious! Hope that helps.
Mellonin is in the same state, but there is something about her that has drawn Thraonwe's attention; he is not yet sure what it is, but it will come clear now during the next night, since Amroth and Mellondu have switched places, as it were. At that point he will have changed his mind and not returned her to Ravion and company, for it becomes clear that there is a connection between Amroth's group and this wandering band of wayfarers looking for a lost brother.
Tharonwe continues to give Nimrodel the same type of messages and dreams as always, but does so carefully, trying to avoid detection by the Elves from Lorien. (Don't ask me what happened to Falowik and Uien - that seems to be a dead issue which we must do without - it occurred to me just now because it certainly would have altered Tharonwe's strategies.)
Nevertheless, Tharonwe knows that he has already been detected via osanwe, and that it is a matter of time before the two parties discover each other..... unless he can force the wanderers back to Minas Tirith.
So the upshot seems to be that Tharonwe will follow Ravion's group south back into Gondor, leaving his swamp and the swamp beast behind. This keeps the Elves of Rohan at a distance, but increases Tharonwe's risk of being caught out away from the lands he controls and knows so well. Hmmm...... would he leave the swamp? It depends on what he can determine regarding Mellonin and Mellondu. So he's not sure yet, and the spinning of our plot will have an effect on his decision (if and how soon to return Mellonin to the others).