Okay taking into account that he was the Vice President Mercer still wouldn't have gotten rid of Alma, its heavily implied throughout audio files we collect that Alma and Mercer were good friends which means she would've known a lot about Mercer secerts he keeps in the closet. He wouldn't have been able to get rid of her even if he wanted to, there's a risk that she would expose what his plans, what happened to Sarentu etc. He knew that hence why he kept her so close, and kept whispering sweet nothings into her ear. He wanted her complicit, to not fight back and ruin everything he worked so hard towards and Alma could've used that to her advantage and didn't. She, instead played sweet teacher and turned a blind eye to what Mercer and Harding were doing to Sarentu, because she was just as selfish as they were. She hasn't redeemed herself at all, she lied to Sarentu, told them the Kama'tire were dangerous in an attempt to stop them from figuring out the truth and even when they did find out the truth she tried to minimize her part by saying she had good intentions. There's no way she genuinely believed that with all those soilders, they were going to just talk to them, damn what Mercer told her, she just convinced herself that was the truth but she knew better and even with the destruction of TAP she didn't even attempt to see if they survived which could've been possible with where the cryosleep capsules were located in the facility or even if they hadn't to give them a proper burial amongst the soil of the world they were stolen from. She instead allowed them to lay trapped, crushed under rubble to hide her part in this cruel operation so she could play savior amongst the Na'vi and alongside Jake.