As you approach each one, a counter goes up (1...6), except for the Titan, and therefore the quest cannot be completed...

Please don't use spoilers. As an alternative, perhaps you didn't have enough time in one day (a day is counted as a calendar day, from sunset to sunrise). It's possible you accidentally clicked "release me from here" on the Titan, which moves it to the entrance instead of the exit. If you're sure everything was done correctly, I can provide the console commands to complete the quest.

No matter how many times I approach the Ent, I always get the same message: "A little later."

So, you've already talked to the tree, and it agreed to go, but the only option available is "a little later"? I checked the code, and it seems I have a bug here. If, during the first conversation, there is no free space in your army for the Ent, the second option is hidden and remains that way. For now, you can only fix this in the console:
@Talk.EnableOption(talk_sea_treant, 5, 1)