I think that the amnesia probably isn't fake, since he wasn't sure who he was when Aqua first found him in radiant garden, and because when he tried to force Terra's heart out, and he fell into the realm of darkness, it probably caused some trauma that made locked their memories away and made their hearts unable to fight anymore (their hearts were shown in blank points talking to each other about who was gonna kick out who)
The first example of him when Aqua first found him isn't a good one. He was disoriented but Xehanort quickly grasped control and it was established the keyblade to the chest caused the supposed amnesia.
Personally I think whether he had amnesia or not is irrelevant at this point. I say this for two reasons:
The first one is the opening of DDD. Whether he had amnesia or not obviously wasn't a question at that point, either the apprentice remembered or MX finally took control which is why MX's keyblade appeared.
The second reason is because of Ansem and Xemnas. These two are confirmed to have the memories of Terra and MX. Xemnas is stated bluntly to have memories of both and we see Ansem's confirmation through his actions such as going back to give Young Xehanort time travel and set off DDD.
Ansem couldn't go to YX if he didn't first remember that he was YX at one point. ;3
No matter how it was initially it still ended with the resurgence of Xehanort's influence and memory before Ansem and Xemnas were even formed but at the same time not long before they was formed.
On topic though:
I meant because Terra is unable to control his own darkness, but Xehanort is very skilled with darkness, Terra as a heartless would probably be easier
for Xehanort to control
This is a very valid point however you poked a hole in it yourself with your post. You mentioned Terra and MX's conversation in blank points.
If Terra had become a heartless then he wouldn't be able to have the conversation. Terra lacks control over darkness and the only known being to hold truly whole human sentience was Ansem who himself was born of a combined character.
In short I doubt Terra would have the control to have that conversation with MX had he turned heartless at that point in time.
That's not to say the Guardian isn't tied to Terra's darkness however. Terra even by himself had an unusual amount of it and then we see it reaching out to Riku after his speech of remembering a "promise he made when he was little".