I didn't see flowering in the last photo, but now I see it was an earlier photo. Clones take best when in vegetative mode, and your plant shouldn't be reverting to flowering with the longest day being 3 weeks away. If you are having difficulty with the clones, you might try using a somewhat larger branch instead of a smaller cutting. It might* help. If it is a branch, I would dip in rooting powder if available, and put it in a small glass of water... about 1 inch. This should allow it to have a little more resilience while it grows roots.
I have grown indoors with 5 or 6 gallon buckets for decades, with good results. A plant will grow as big as it's root system will allow it to. A 15 gallon pot will allow a lot more root mass and a much larger plant, to a degree. The plant still has to have enough time to grow large before flowering outdoors. The larger pots come in handy only if the plant has enough time to fully take advantage of it... shorter growing seasons might not have as much luck, but as a general rule, plants, like aquarium fish, seem to grow bigger depending on how big of a container they are in. Indoors, I can see the difference between identical clones grown in a 5 VS a 6 gallon bucket. I'm sure a 15 gallon container would have much nicer results outsoors!
