One consensus is that Carvana/Vroom/Shift are buying cars at a loss to try and capture market share. Carmax has more of a track record so they might be getting a better shake like your experience. It could also be you calling personally that unlocked the lien, where the other companies don't work that way.
Some manufacturers like Volvo aren't letting you out of the lease early unless you're leasing another Volvo.
They absolutely are. Where I live, these companies are (and have been before COVID) paying well above market rates for newer used cars from private sellers. On the flip side, all of your typical auto sales rags and sites that used to have listings from private sellers and used car dealerships are now mainly carvana listings.