Lunges are a good lift to supplement your squats and are great for your quads and glutes, but they don't
replace the squat. No, they are not viable.
Mixed grip might take 2-3 sessions to get used to. I have a friend who is the same way- can't deadlift with a mixed grip. Just switch one hand to a suppinated grip like you're doing underhand rows.
or just put one of your hands up right now like you're doing a
chinup. you might look like a dumbass at first
Then, bring your arm down like the top part of a curl, then go down like you're bringing the weight down to curl it back up again. That's how your hand should be and yes, i just made that up this second.
Or just reverse your hand.
Hopefully, one of these explanations will suffice lol