What "giant signs"?
The X-Wing helmet? The doll?
Red herrings simply meant to distract us and have us guessing.
Nothing in the film was any states that she HAS to be the daughter or relative of any previously known character. Quite the opposite - they made a point of telling us that Rey was abandoning her past, of Maz telling her family was never coming back.
Why would Luke abandon his own child into poverty and practical slavery rather than leave her with his sister or with people who would love her? Rey is old enough in the Force vision to remember who her family is - that the "myth" (in her own words) is her father or her idol (Han Solo) is her uncle.
And why does neither Leia or Han comment that Luke lost more than his Jedi apprentices and nephew? Why does neither Han or Leia think that this girl has the same name as their "missing" niece and matches her age? Why does the film end without Luke recognising her?
In the film (I don't know about the novelisation) Rey never gives specifics about her family - both parents or just a mother & siblings - and it isn't beyond the realms of possibility that Luke had a child he didn't know about but the fact is that effort was made to point out that Rey was letting go of her past and looking to the future - to suddenly just alter that to make her a Skywalker just for the sake of it makes no sense.
while it's true no specifics were given, there were several "big clues" that point to Rey being a Skywalker.
1.
The Skywalker Saber
They made a specific point to highlight the lineage of the saber ( it belonged to Luke, to his father before him, and now it calls to Rey ).
They didn't say that the sword was passed down from master to apprentice ( Kenobi to Luke to Rey ). They made a point to highlight the generational lineage of the sword - the Skywalker family.
Also, Rey's force powers started to awaken when she was in close proximity to the sword. and when she touched the sword, she experienced visions of the past and future that involved Anakin/Vader, Luke, and herself.
And, in the final battle, when both Kylo and Rey used the force to pull the sword from the snow, it chose Rey.
There's also parallel symbolism in how Kylo offered his sword to his father but then killed him - suggesting Kylo was unable to let go of that power and the lure of the dark side. Contrast that to Rey who freely offered the sword to Luke - suggesting she has no desire for that sword and power for herself.
2.
Rey's "Safe Place" Dreams
Another big clue is when Kylo probes Rey's mind and sees her "safe place" that she dreams in her mind to escape the lonely deserts of Jakku.
What does she imagine in her mind? An island in the middle of an ocean.
And where do we find Luke at the end? An island in the middle of an ocean.
3.
Too many similarities to Luke and Anakin
There are also way too many similarities between Rey and Luke and Anakin.
All 3 grow up on a desert planet.
All 3 are gifted pilots and mechanics.
And by the end, Rey "inherits" the Skywalker saber, the Falcon, Chewie, and R2.
So, to me, it seems they went way out of their way to drop these hints that Rey is a Skywalker, and most likely Luke's daughter.
And as for your other points:
1. It's possible that Rey isn't even her real name and some of the supplementary material suggests that may be the case ( the name on the pilot helmet she has on Jakku is a name similar to Rey ).
2. I agree with you that it would seem out of character for Luke to leave his daughter on a hostile planet in the care of someone like the junk dealer. However, it's also possible that someone else could have left her there. Someone like Kylo Ren. He seemed to know more about her than he was letting on. Namely, when the officer told him that BB8 was with a "girl on Jakku," Kylo immediately snapped and asked "What girl??!" Like that triggered something in him.
3. As for why Rey doesn't remember who Luke is ( or Han ), it's possible she could have had her memory Force wiped. And again, that could have been done by Kylo, who was shown to be proficient in the mental aspects of the Force.