OK, let's answer the easy bits first...no, you can't control the response curve with a modulation signal. But then, that's not a really common musical usage, so you're not likely to need all four of the VCAs implemented in that way. But the response curves DO control response rate...
The difference between linear and exponential, math-wise, is the difference between 1, 2, 3, 4, 5 and 1, 4, 9, 16, 25. Very related (the second is simply the squares of the first) but not the same, function-wise. When translated to loudness, attack/decay values in linear VCAs rise and fall with a direct, linear relationship to the CV. Go up a volt with your CV, the output of the VCA goes up a volt. But with exponential VCAs, attacks and decays occur more abruptly...at least, as far as a voltmeter is concerned. Since we're talking human hearing here, though, the exponential response of the VCA is a closer match to how our hearing perceives the attack and decay of acoustical instruments, while the linear VCA would just sound like turning a knob up and down UNLESS it's fed with an exponential modulation signal source, such as an exponential envelope gen. So, when you change the response, you're adjusting this factor from the linear relationship to an exponential one, and thence everything in between. Now, why you would adjust this...OK, consider percussion instruments for a sec...
Take a drum...any drum is fine. Hit it with a stick, which gives you the hardest attack. Now a rubber mallet. Then a yarn one. And after that, a soft mallet (the fluffy sort). This is the sort of result you get from turning the response on a VCA from exponential (hard attack, fast decay) to linear (soft attack, easier decay). It's an odd effect, and while some VCAs can do that under CV, most don't. But if you really need this effect, it's actually simpler to send the VCA a modulation signal from a mod source that allows you to shape ITS curve.
Now, as for the Intellijel module...yes, it can act as a mixer. Or two mixers. Or a mixer and a VCA (or two). Or four VCAs. It all depends on how the OUTPUT is patched.
Since the Quad VCA uses an normalized but interruptible mixbus, you can patch an output from OUT 2 and also 4/MIX, and this would give you a pair of 2-input mixers. Or if you need just a single VCA for a certain function, you can take VCA 1's output alone, and then use the 4/MIX output for the sum of the other three VCAs. This is VERY useful if, for example, you have only a couple of audio sources that need summing, but you want some interesting modulation behavior that goes to some other parts of the rig. For that, you'd split out the first two VCAs via their dedicated OUTs, then use 4/MIX as a sum for the audio coming into IN 3 and IN 4. And, since you can change the VCA response, VCAs 1 and 2 in that example can be set to function linearly, while the audio in 3 and 4 can have the necessary exponential response. Versteh'?
It's also worth noting that the Quad VCAs CV inputs work in a similar manner. You have a normalized mult behind the panel (of sorts), so sending a single CV to the top of the input bus will affect all four VCAs simultaneously. But you can also patch different CV/mod signals in with the same sort of arrangement as you find on the module's mixbus. Just remember that these patchpoints have a "priority" to their mult behavior: bottom to top on the CV inputs, right to left on the mix, and any patchcord inserted at the lower/leftmost points will split the mult. Want the same CV on 3 and 4, but not 1 and 2? Simple...send 1 and 2 their own CVs, and send the paired 3/4 a single CV via VCA 3's CV in. Done!
Oh...also, keep in mind you don't have to mix ONLY audio with this (or any other DC-coupled mixer). You can also create complex, composited modulation curves by mixing mod and/or CV signals, and yes, these can also be under VCA control to gradually change voltage levels at the output. This is where the fun starts...
Better?