Everyone in that villa feels sorry for him. No one is gonna come out and say that he has no personality because they'd be seen as a dick on. And you say that we only have 45 minutes to perceive Alex, yes, but we've also had 45 minutes to perceive the rest of them, and every single person in that villa clearly has a good personality from what we've seen in those condensed 45 minutes, but Alex doesn't.
The Ellie and Alexandra situations were very similar. Ellie before the recoupling DID express her frustrations and concerns to Alex. She told him that he and the rest of the villa were placing too much pressure on her and that it needed to stop. Alex continued to pile on the pressure AFTER she warned him about her not liking it, so she lost interest. Alex however lost interest in Alexandra for a more selfish reason. He clearly had concerns over his relationship with Alexandra but instead of expressing them before the recoupling like Ellie did with him, he suppressed them UNTIL AFTER the recoupling and even Alexandra said that immediately after the recoupling, Alex turned cold with her. That is exactly what Alex was accusing Ellie of yet Alex did just that. Like you said, Alexandra spent 24 hours a day in there with him whilst we only see 45 minutes, and she herself said that he turned cold with her immediately after the recoupling.
He was the one person in the villa who still called Adam a stand up guy. Every other guy at some point, including Jack, Wes, Sam and Eyal alluded to Adam in a somewhat negative light but Alex never questioned his disgusting tactics. And yes, Alex does have a 'nice guy' mentality. He feels that just because he comes across as polite and well mannered that any girl who he likes should be grateful to coupling up with him. He truly exposed that side of him with Ellie.
Love Island's Twitter, Caroline Flack and the show's voiceover guy have all referenced girls being brought into the show for Alex so that isn't me making something up because it's been alluded to by them. And yes, Samira and Alex coupled up as friends, but so did every other couple. None of the couples knew each other at the start so they all had to get to know one another. Alex just failed to talk to Samira properly which is why he got friendzoned. And with Megan, she herself said that she had more fun with Eyal, which is why she chose him. Which goes back to my first point that he has no personality and is boring. Megan literally said that in the nicest way possible to spare his feelings.
Finally, Charlie and Grace's situation really showed who Alex really is. Seemed head over heels in love with Charlie, and for NO REASON whatsoever, he suddenly went and started to chat up Grace. For no reason. He seemed head over heels for Charlie and then went for Grace, and even when he slept with Grace in the same bed, the camera showed clearly that everytime they got into bed, he made no effort to talk to her and just turned over.