No, you cannot use two single-channel memory sticks together for dual-channel operation.
No, you cannot use two single-channel memory sticks together for dual-channel operation.
one stick of RAM operates as single-channel, but it switches to dual when paired with a compatible second stick in the right position. Yes, that should function.
No, they wouldn't. Two 1x8 pieces and one 2x8 piece produce different configurations.
Yes. It would operate at the lower frequency of the two sticks, meaning both 3200 and 2666MHz would function at 2666MHz.