They are soldered in place from the factory IIRC. If that won't work; then use some epoxy and then solder.
No; it supplies the bias.
IMO - the 457 is better than the 458. Not that the main circuit board is any different, but it has two meters on the front and other useful hardware added that the 458 doesn't have. But it is forty years old and recapping it is a must. Once restored it is every bit as good as a Cobra 2000GTL - if not better - IMO.