"the end user must be able to replace the LGPL component" seems to be too one-sided IMO.
The user must be able to get the source, modify it, and make his modified version work with any (proprietary) application that chose to use the LGPLed code as a component. For that to be possible the proprietary application's authors must distribute their software with clear documentation stating which LGPLed components were used and provide source code and build scripts to rebuild these parts and relink them with the rest of their application. They also have to give you all the rights under the LGPL to further distribute and use any of their modifications to the LGPLed code for any purpose (not limited to use with their application).
Generally?
Not without permission. They are allowed to place further restrictions on the non-LGPL pieces, as long as they allow the two things they have allowed.
LGPL 2.1 is a weird license.
The section that allows for object code release does not require object code release under any particular license, it only requires that the LGPL portion stay LGPL.
The user must be able to get the source, modify it, and make his modified version work with any (proprietary) application that chose to use the LGPLed code as a component. For that to be possible the proprietary application's authors must distribute their software with clear documentation stating which LGPLed components were used and provide source code and build scripts to rebuild these parts and relink them with the rest of their application. They also have to give you all the rights under the LGPL to further distribute and use any of their modifications to the LGPLed code for any purpose (not limited to use with their application).