No worries, I would suggest trying again. For your 2nd attempt, see if you can purchase seedlings which have more branches starting down lower at the bases, and try not trimming the roots much at all, and pot the project up in late August. Trim the foliage during the repot, but only do it once. It is a lovely species to try, so don't give up. You could trial the project with more advanced stock,.... or perhaps slip pot the same size seedlings into a slightly larger pot and wait till they are a bit more advanced and then attempt another group planting next year. But that requires a lot more patience.
Alternatively, if you don't want that instant forest look, then get a deeper pot, and just basically slip pot all the seedlings into a larger pot, so there will not be any severe disturbance of the roots. Just prod the edges a little to encourage the roots to grow out and not back inwards. Even though you will have a deep pot, once the group setting takes shape, then you can start to reduce the roots in time. Besides, this will encourage thickening of the base rather than instantly putting them into a shallow pot. You can even just use a styrafoam (not sure of the spelling on that one) container, until they have really settled in and thickened up. With group plantings, remember to rotate the pot once in a while, to distribute the sun exposure all around.
However, if you want that instant look of a forest and are worried about root trimming or pruning, try cunninghamiana or glauca. They are much hardier and will tolerate more brutality. When I repot Littoralis or Torulosa, generally I will cut back less of the foliage/roots than I would with cunninghamiana or glauca. But everyone has their
This was just a group planting of Torulosas that I started to test sun level exposure to this species Torulosa. It was slapped together very quickly on the 23rd March this year just to see how they hold up. In preparation for the pot, I have a washing bucket full of water, and take the seedlings out of the packets, then dunk them all into the water, then let them sit in the water for about 30 minutes. This does 2 things... it keeps the roots moist while you are repotting them up, and it makes the soil easier to gently pull apart and get the roots exposed so it isn't so curled into itself and you aren't yanking on the roots too hard so that it dies. Then one by one, I take each out of the bucket, and it takes about 10 minutes for each one, to slowly but surely, pull out the old soil, ..... however, I generally do not touch 3 or 4 cm or so at the top of the root ball, where the base is. If it looks like it has thick feeder roots at the top, then I might get a little eager and cut about 1/3 off the bottom of the soil section. Then, I just place each one into a pot, (make sure there is already soil in the pot so the roots are well covered, and repeat each one until you have them all in the pot. Then fill your pot up with soil. Then slowly with, say a hand water sprayer, and put it on halfway between mist and jet, then spray this right near the surface to ensure the soil doesn't have large air pockets that will collapse over the next few weeks. You basically want to try and get all the seedlings in firm, so they aren't going to be blown over in wind, or when being watered. Use a well draining soil mix, and there you have it.
You can see from the pic, that they have all shot out new growth since then. They are only watered when almost dry, and get probably 3-4 hours of filtered/full sun a day. They would prefer more sun if I could offer it though.
Sorry 
