do you think Lexa would have betrayed Clarke again like she did at Mount Weather if she'd survived ep7?
No, she wouldn’t have.
Lexa’s entire arc in season 3 is about regaining Clarke’s trust and working towards peace and unity. Grounders and Skaikru form a reluctant alliance in season 2, but the “us/them” mentality is still very much in place, and then Lexa’s betrayal fractures whatever progress they’d made. Now in season 3, what is the first plot point we’re introduced to with Lexa’s reappearance? Her desire to have Skaikru join her Coalition. In Lexa’s words
It’s entirely different from the alliance in season 2. Skaikru becoming the thirteenth clan in Lexa’s coalition means that they effectively become part of her people. Which means that A) they would be protected against external or internal attacks since Lexa’s main responsibility as Heda is the safety of her people, and B) there would be no point in a betrayal because they are part of the Coalition she created and swore to protect.
What did Lexa say to Clarke after she took the deal at Mount Weather?
“I do care, Clarke. But I made this choice with my head, and not my heart. The duty to protect MY people comes first.”
With her offer, she is making Clarke’s people her people. She is transcending the tribalism that has been the cause of most conflicts ever since the beginning of the show in favor of unity, stability, hopefully peace.
And this is only the political aspect of it all. Because you have to take into consideration Lexa’s feelings too. And this is one of the reasons I love this character so damn much: all her layers. She is smart, pragmatic, even ruthless on some occasions (as we saw), but she isn’t a robot. She feels so much, probably more than she would like since it caused her so much pain, and her feelings for Clarke are definitely one of the reasons she suggests that Skaikru enter the Coalition. Don’t mistake this for Lexa letting love cloud her judgement and making her do irrational things for the girl she loves. As we saw earlier, there’s a great deal of logic in Lexa’s suggestion. Honestly, this isn’t about romance at all. First and foremost, this is about Lexa having to do something awful (the betrayal) and not wanting to do it again. But could she promise Clarke that she’s never going to betray her again if her people always have to come first? No. So, she thinks about the one solution that allows her to both be faithful to her duties and, if not make up for what she did to Clarke, at least be in the position of not having to do it again.
This is what her vow is about really.
She can’t erase the past, but she is promising not to do what she did again. No more Trikru vs Skaikru, just one people. And no more betrayals. Sure, this last part is more personal, but nonetheless important. If anything it’s the most important aspect of Lexa’s vow, how personal and intimate it is. This isn’t just one leader swearing fealty to another. This is one person vowing to do anything in her power to not hurt the other person again.
Lexa literally puts her life on the line to stay true to her vow and push towards unity and peace, she eventually dies for it. So the core of it all is no, she wouldn’t have betrayed Clarke.