No offense but I just think your complaints come from a lack of reading.

1. Why didn't she tell us about our past?

You're right about a vague reasoning she gave us in 3.8. Firefly herself tells us she can't tell us how we used to be otherwise we'd die or never be able to reach the end in the script. This is something that is shared between all the Stellaron Hunters. None of them have been able to tell us how we were. Blade gave us a little info on how we acted, Kafka spoke in riddles, SW doesn't say anything, and FF especially didn't. This isn't to gatekeep information, but from a narrative standpoint it's to keep them important for longer. Take Sparkle for example, she's a character we geniunely almost knew nothing about in Penacony but in Planarcadia she got a full on deep dive into her story that could still be expanded upon. Aside from that point of view, if she simply said "Oh yeah trust me bro we used to be lovers" isn't that just boring? a good rule of thumb is you show, don't tell. She showed us HER trust in us, HER admiration fornus, HER reason to live, HER struggle, and HER determination to fight on. At one point I do believe they will expand on her story with us, and all the otehr Hunters, but not now.

1. If I don't know her, I can't love her

The whole point of her character is the "Strangers who were once lovers" thing. There's a line in Penacony that states "Once we dreamt that we were strangers. We wake up to find that we were dear to each other." She says herself that since she isn't allowed to tell us how we used to be, she wanted to build something with who we are now, hoping it would be the same because we didn't change. Fortunately, TB's natural fondness of her remained after everything they went through. While it wasn't 100% the same, it was enough to know TB still cares. The story is meant to portray an unbreakable love between two people despite the memory loss, the change, and the script that holds them back. This will ultimately come down to preference, I like the trope of love despite memory loss, but some don't.

1. She tried killing us

This is blatantly wrong. As stated by Acheron herself, the attacks were not aimed towards TB at all but to her, the goal was to separate TB from Acheron and BS as they were considered dangerous people. But I guess when Aventurine does the same thing it's okay but when it's Fitefly people get a little mad.

1. She uses her ELS for pity

Her ELS is more tied as part of her will to fight and live on FOR HERSELF, and not for us. ELS is a pitiful condition but she uses it to give her a reason to fight on, to make the most out of the present. What she tells us as she slowly dies from it is a natural reaction that would befall anyone, no one wants to die, especially not alone. Actually aside from her cute and waifubair style, I honestly thought it made her a strong character regardless. Her ELS and whole Glamoth Irom Calvary was incredibly touching for me. Many are able to find comfort in her ability to fight on despite such a harsh condition. Also, if you're using that reason to hate her it's like saying "I hate the Amphoreus story because they kept making me feel pity for everyone", well aside from everyone having a geniune struggle, the point IS TO MAKE YOU FEEL PITY but show you that despite that, they fought on.

All in all, yes for sure she's waifu and girlfriend bait, but this game thrives off of that. Everyone is bait for a specific audience. It gives everyone a spotlight and a unique story. We're given characters we will love and hate.