use this calculator http://www.ukpower.co.uk/tools/running_costs_electricity/ for say, 200 watts run 18 hours per day. i think you'll find that the electricity is cheaper than the bulbs.
also, i should have said earlier that there is a lot to learn about growing weed than just lighting. this...
you did exactly the right thing. it will help going forward to have a small fan blowing on the stems - it makes them stronger to be able to support a large plant.
as everyone else has said, you need more light but they haven't helped you figure out how much is enough. weed requires 3,000...
if i understood your post correctly, you have set the ac for below 81 degrees but it cycles off at 81 (tested with an independent thermometer?) , then that's your test there. you probably need a manufacturer's rep to fix it.
no reason it wouldn't work. probably obvious to you, but you want your carbon filter at the top of your shed, preferably on the opposite side of the shed from the a/c.
there are two main choices - batwing and air-cooled. if you're able to keep your closet door open and you don't have an inline fan handy then i would just go with the cheap batwing. brand really doesn't matter.
that's about 14,000 lumens and should be ok for one plant. you need the right spectrum for the period of growth, however. 6500k for vegging and 2500k for flowering.
a 600 watt light would give you about 10,000 lumens per square foot, which is excellent, but you may not be able to handle the heat. if you are living in a situation where stealth is not important then keeping the door open with sufficient fans would work for either a 600 watt light or a 400...
sorry, you will get nothing with that light. weed is kinda particular about the kind of light it needs. if you're just fooling around and trying to grow a little i would suggest compact fluorescent lights (CFL). if you've decided to make this a long term hobby then i would suggest you invest...
the close up pics of colas look really close, maybe a week. check the trichomes if you have a microscope. the wide angle shots don't look as ready, maybe 2 or 3 weeks.
the first thing you should do is lower the light(or raise the plants) so that the top of the plants are within a couple of inches of the lights. this will give you more help than reflective walls. then if you want you can create your own reflective wall, even out of something as simple as...
your problem is going to be height (don't think you gave height?). count on 6" for the pot and 6" for the lights and the rest of the height is what you have left for a plant.
never tried short rider, sorry.