I would just do 24 hours if I didn't have a timer, if I did I would go ahead and mimic the light cycle. They won't flower going to 15-16 hours of light.
I would go with the 6 inch with speed controller if it was me. The way you're wanting to run everything would probably work; although I prefer to exhaust my grow area and run my cooltube with separate fans and separate ducts.
I have a cooltube with a 400w in a 3x3 tent and it makes a huge...
I don't think you're having heat problems at 70-75 degrees.
By slow water uptake do you mean your plants aren't drinking enough water? How big are they? How often do you have to water?