I have had good luck with the multipacks and they do sometimes appear rootbound. I now just plant each one (separated from the others) as it is, in loose soil and water well, and while I'm sure spread out roots are better, the plants seem to grow and perform. Of course if they were planted in clay I would expect them to not do as well. But much of my patio planting is in large pots with good soil mix lightened by perlite.

Rinsing dirt from roots helps the process of gently pulling them apart. I would not soak roots -for more than a few minutes that can kill them.

I wouldn't stress the roots too much and as Arcy said, soaking for more than a few minutes could lead to rot. Cosmos prefer well drained but rich soil. They should do fine.

You want to prevent circling around roots so the traditional way is to slash each side so you can loosen the quarters so new roots will come from the center middle. If you have that matt of circular roots on the bottom you can trim that off but mostly you just want to release the center area. Not too much massaging. If it isn't too bad you just drag a fork across the surface of the sides to break off the outer roots. Worth trying. Don't overwhelm with fertilizers and such.

patriciae

"You can be pretty brutal with the roots. " IDK about that. The mantra at the green house where I spent many early springs planting seedlings was "hold them by their ears, gentle with their toes". Deer nipped off all the flowers + from two of my inpatients and begonias a couple of weeks after planting. Right now they are in full bloom. If it had been their roots they would be GONE. Of course roots cannot rot in a few minutes...but to think you know which plant can handle how much soaking well better to be cautious....

Been doing this professionally for the last 30+ years. Transplanting seedlings is one thing - nursery starter packs are entirely different. The roots are actually the most durable and regenerative part of the plant. They are created and replaced constantly.