You can control the volume independently, as you said. You could make a macro which would turn the tv speakers on and turn the receiver off, and another one to reverse the process.
To find out about discrete codes, see this thread
[Link: remotecentral.com] It will answer most of your questions. The 098 code is not actually a discrete code for the speakers because it works as a toggle code, which means it turns them off if they are on or on if they are off. Unfortunately there are no discrete on and off codes for the speakers.
I don’t see a problem with just using the receiver audio, but that is a matter of preference. Using the receiver, you can make macro’s to switch your tv input and your receiver input, so it shouldn’t be a problem for your wife.