My outdoor plant is too small due to errors and st...

Yes, is the short answer, it would be a problem. You only have 10 hours of daylight according to your diary.....just start a new plant when Summer begins and let this one just be what it is going to be.

I agree with numbers.

is it a photoperiod plant? If so, it flipped to flower based on length of night and nothing you did. If it is a photoperiod, no do not give it 18hours of light. That'll just fuck it up in a really bad way. if you re-vege it, the length of time to recover depends on how far into flower transition it got. Further in = longer recovery time. It'll grow like shit for 1-4 weeks. It'll branch so much it'll be a total dumpster fire that you'll have to spend a lot of time pruning to thin it out or else you get 100s of popcorn size buds. re-vege sucks. Avoid it at all costs. you'd be better of letting it go and if yield is going to be insufficient, pop another seed now. A shit load of branches is called "monster cropping" but it's retarded name because it doesn't increase yield, but does increase your trim workload exponentially.

If I stunt a plant, even in veg, I scrap the plant. It is often not worth going through the stress. The effects of the stress often linger. Even when I do reveg something in flower, I typically take a cutting into an aerocloner then take a cutting off that as soon as growth looks normal, then throw away the original cutting. People do all types of permaculture and perpetual harvesting type stuff these days, I personally wouldn't though.