How can I record music in computer at the same time as sound from external mic?

I'm a long-time PBS announcer who wants to record shows from home. I need to be able to switch quickly and efficiently between voice (using Blue Yeti) and music (using thousands of files stored in my iTunes library). Recording commentary over the music (sound on sound) would be a big plus (but not essential). Here's a longer, more detailed description of my problem:

[Later: An ABSTRACT of what's below: Can I "efficiently" record music already "IN" my iMac (iTunes) at the same time as I'm recording--into the same software program (Sound Studio)--my voice from an "EXTERNAL"microphone (Blue Yeti)? Of course, I'm recording the program on my iMac, then saving the finished file to Apple Lossless which will be burned to CD using iTunes.]

I'm able to get great recordings from a simple program (Sound Studio) offering precise wave editing (impossible with Roxio Toast). But now I'd like to do from home what I've done for many years at our public radio station--viz. record programs with voice and music. Being able to make instant switches from music to mic would be acceptable--but even better if I could also talk over music. My mic is the Blue Yeti, which comes close enough to the vocal quality at the studio (providing I use a pop filter). But I'm unsure about the best way to do the switches between music and voice let alone voice over music. I've considered all of the following:

1. Garage Band (a steeper learning curve (for me) than Sound Studio, but if it has the capability (of recording iTunes and mic w sound-on-sound), I'll start working on it.

2. Using a simple interface (Behringer UFO 202 has worked nicely for transferring external sources--LP, cassettes, FM radio) to Sound Studio on my iMac.

3. A mixer (with slider switches--which I use frequently at the radio station). Tried the $50 Behringer Xenyx302USB, but sliders are tiny sticks, inaccessible, dicey operation). If I double the price, I can move up to the Xenyx 1002B (more bells and whistles, preamps, inputs, etc., than I need for my $100, but the slider switches are bigger and even have finger-friendly covers (rectangular knobs) on the end of the protruding "sticks"). Or the Behringer Pro Mixer VMX100USB (same price for a reassuring "mere" 3 sliders--the most I use in the studio). It's billed as a "DJ Mixer," which I am in a way--for PBS.

4. An external CD deck--if the simplest way is to record from music, like microphone vocals, from an external source (though It's getting hard to find good, reliable, external CD decks (I don't think Apple's SuperDrive or the USB player-recorder drives are the answer--it needs to be solid and running from AC).

5. Two computers. My IMac (late 2012) plus my Macbook Air (summer 2013). That way I can record iTunes from an external source (like my mic) into my program. But can I get around the business of opening system preferences and resetting the input source for each of my external sources (Mac Air and Blue Yeti)?

Thanks in advance for reading this (or some of it) and even more for any advice, recommendations, do's and don'ts. (I'm starting to realize that, with a slider mixer, my main question might be this: can I record "ON' my iMac (using the software GarageBand) the music that is already ("IN") my iMac (in iTunes) while at the same time recording from a mic plugged into my iMac?