Photoperiod requires the correct "maturity" (really just the build up of flower hormones) and the photoperiod associated with flower (10 or more hours of dark depending on lights, usually 12) before it can flower from seed. Clones are different. If you had a seed in less than 18 days from sprout starting to produce calyx clusters, I'd be very curious. While it may be possible it seems very highly unlikely. Usually they are barely starting to show sex at 2.5-3 weeks.
In soiless or well aerated soil you get air pruning which is essentially topping the roots.
Hydro plants don't need all the roots. The more roots there are, the more surface area to absorb nutrients and water. Which means more plant matter, higher supportable yield, and quicker growth. If you lose roots, what remains will work in overtime. You will lose yeild but not exactly proportionate as you can literally grow 2/3 - 1 oz with about a solo cup sized hydro rootball and good grow conditions. Transplanting does help even if you don't push the waiting period, usually I see about a doubling in size in about 1.5 weeks. I guess I'm used pushing my small pots to get most explosive growth. I don't get rootbound because I also leave an inch or two of perlite on the bottom to prevent overwatering. Literally looks like a hydro root system with hundreds of spitting roots when you look at bottom of pot. I wait until pot dries out in 2 day or less consistantly before transplant.
At op, what is the plan with the 7 gal pots? Are you only doing one or 2 plants? 5 gallons and up tend to take forever to dry out. Personally I love the Walmart bags as they are a pretty appropriate size for single light and I can put up to 9 under my 600w. Filled up its somewhere between 3-5 gallons.