People want to see Chell for many resons. They really like Portal and want to see a crossover into Half-Life, They like Chell and consider her Gordon...ish. She's silent and kickass and the way she solves the puzzles shows majorz intelligence. I think she's cute but thats just me. And I'm not saying she has to be a hudge frekin part of EP3 or HL3 like Alyx or something I'd take a small cameo like her walking by while Gordon goes down an elevator just so I can have a sence of satisfaction that she's still alive that my hard work solving puzzles and getting out of there paid off and I think many players want that.
Black Mesa focused on teleportation bewteen dimensions. Aperture Science focused on teleportation within one dimension. So I don't think that Aperture Science's technology will help the Combine to get to Earth, but it might help their mobilisation once they're here.