At the time of the D1 manual, when they wrote that Inarius' wings were stripped, I'll bet they had in mind typical feathered angel wings, not the energy tendrils that showed up in D2. I think there's an unused graphic of an angel statue that looks such. And of course the iconic angel on the mana globe was always the more traditional style. In other words I'm not sure they thought through how to tear out the energy wings and whether they want to keep that part of lore.
A summon is to call something from somewhere else. Lilith s more than the sum of the "ingredients," that would instead be manufacturing her.
Its possible/likely that she needs some kind of human host to tether herself to Sanctuary, just like all the Prime and Lesser evils had. However while host is shown to have some influence on their power (Diablo as Albrecth vs Aidan), its also clear that Diablo's power clearly surpassed any one host.
Lilith is causing the remaining humans to act worse and spreading chaos and corruption by here mere presence. She may also be summoning demons to help her out and that is causing massive collateral damage to humans. Some heroes might want to get rid of her for that reason alone, even if it isn't the best thing for humanity in the long run. Remember the lore panel stressed that feeling of how the good guys don't always win or they arrive a day late and a dollar short, whatever we accomplish in D4 is almost certainly going to have a huge failing or downside. Like maybe killing Lilith opens us up to worse aggressors in expansions.