You are looking great so far.
You might cut down on your veg time. They will Stretch after you switch to12/12.
If you are going into the three sqiare foot cab.
With screens to tie it down.
250 watt hps will get hot. Ypu will need 18 inches from the top of the plants and the light.
How tall is your cabinet and your pots?
If that is 3ft. wide by 2ft deep and 6 ft tall. It should be great for 250 watts.
Your plants will naturaly want to grow towards the light. Make yourscreens fit the profile of your light's footprint.
The extra space will help with heat issues.
My first grow was one plant in a 1 gallon pot under one 100 watt bulb hanging next to my water heater. I never have used a screen.
Yes.
Take a clone from each plant. Then if it turns out great ypu will have a mother to start future clones from.
Sounds like a plan.
As far as ventilation issues, could I get away with just using fans and good circulation?
Not sure about that. I But not likley.
At the very least it would need a vent in the top and bottom. 250 watts is a lot of heat.
Plants need fresh air too.They suck in water and breath out water. The humidty would go up without air exchange.
What 250 do you recommend? Should I go for a combo HPS/MH unit or just a standalone HPS?
Good Luck.