I was always afraid of the rolling over while swaddled too, she just turned into a houdini before she could roll over in the thing
It's not too hot, we live in Utah (Desert), and I've been using it since August. We do keep our house pretty cool, but I just put her in it in a onesie, or only her diaper if it's warmer than normal. She's never been sweating when I get her out of it at all. Supposedly the inside layer is some sort of breathable something. It''s not like solid flannel or wool.
It works because the arms are pretty short, with a larger armpit area, so it keeps her arms down and puts a little weight on her, so she can't startle herself awake. It restricts her movement enough to keep her asleep, but when she's awake, she can move around in it reasonably well. She can get her hand in her mouth and stuff if she's really determined. Because of the position it keeps her arms in, I don't think she'd be able to roll while in it, but Kaiden is probably stronger than Josie is. Even if he were able to roll over, he should be able to roll back if he wanted to, but I have no experience with rolling in sleep because she doesn't do it yet

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I found she needed the swaddle desperately, there is no way she could have slept with nothing. It has definitely put her back to sleeping better, as well as an added nap during the day, which I could never get her to take even with the swaddle. Let me know if you try it on Kaiden and how you like it. It has really seemed like magic to me