I regret not reading this post when it was first posted, but you noted "first hand experience only", so I stayed away LOL.
There's nothing wrong with these dyesub-to-cotton transfers. People just get upset when others believe it's true sublimation when it's not. It's the same concept as using Heat Transfers for cotton, except the dyesub-to-cotton transfers sublimate the inks to the transfer fibers and then the transfer adheres to the cotton, whereas, Heat Transfers use pigment inks (recommended) to dye the transfer fibers and then the transfer adheres to the cotton.
The only difference is the dyesub inks permanently adhere to the transfer's fibers, whereas, the pigment ink does not, and over time pigment transfers will fade. Well, another difference is most Heat Transfers need to be trimmed and the light dyesub-to-cotton transfers don't.
But they BOTH use a transfer that adheres to the cotton. Whether one lasts longer than the other (long term), is still in the air. Depends how the garments are washed. Although, I use JetPro Soft Stretch with pigments inks and have created many designs for my wife's shirts and still look great, well after a year.
Sidenote: Most of these dyesub-to-cotton transfers (that I've read), recommend using Vector art only; not Raster. Dyesub-to-cotton transfers have trouble transferring raster gradients or lighter color areas. They need full-toned colors. Regular Heat Transfers can use Vector and/or Raster art and transfer with no problem.
As far as your concern for the breathability for babies, cotton is the best choice. MOST babies don't drown in sweat, so they don't need polyester to wick sweat away. And if a baby was to sweat in a polyester onesie, I bet they'll be cold because MOST polyester has a tight knit, which will keep the sweat close to its body. It's not like they are out running around where the sweat can wick out and dry fast. LOL
So you think cotton is more breathable for babies? There is your first mistake.
Second maybe there is a reason there are not reviews for Forever or any of the other so called solutions to dye sub on cotton. You would be lightyears ahead using Vapors Onesies.
First off, MOST cotton is more breathable than polyester. Depends on the knit used, but cotton has a more open knit than almost ANY polyester.
Cotton is not breathable. There is a reason virtually every sports attire is NOT cotton. Now even sheets, pillowcases etc are moving off of cotton. Vapor Apparel is probably the top dye sub apparel manufacturer and have onesies.
Secondly, again, cotton IS breathable. The reason sports teams moved to polyester jerseys is because they stretch helluva lot more than cotton and cotton/poly jerseys, they are lighter in weight and they wick the sweat away from the body to the outside, therefore, dries faster. Cotton on the other hand, is heavier and when you sweat, it absorbs the sweat instead of wicking it out, and dries a lot slower and makes the jersey heavier. Believe me, I still have original baseball jerseys that were 100% cotton and when I used to sweat, woowee it was harder to move, got hotter and felt heavier. I also have these newer polyester authentic jerseys, and they are awesome. The only negative is they can freaking snag and rip easy. The older jerseys started to transition to open holes in the cotton/poly, and now they are like an open-knit mesh polyester to help with breathability.
I believe the reason people think they are more breathable than cotton, is because when you sweat, it keeps you cool, so you think air is flowing through the garment when in fact it's your sweat touching the skin. LOL