I think in general you can't expect to get very reliable mixing unless you use an active mixer, eg like this:
and even then you may not get great results unless you buffer each output, not sure how the 40106 behaves. I am not sure at all why going from 5 to 6 mixed sources is creating an issue in your case. In terms of doing a simple LFO, just raise the timing capacitor of the oscillator in question until the frequency is sufficiently low. You can then feed the output to an LED and use it to modulate a CDS cell like you're suggesting.