Edit: Welp, I tried to use process plant to mill the silk roses but it didn't work. Even tried using the process to x ones and nothing happened. Going to try a millstone and see if that solves it.
I believe there has been some misunderstanding. Silk roses are not millable. You can only process them at farmer's workshop. It gives you thread and seeds. Seeds can only be planted again and thread is used in clothing. That's all. Perhaps you mean some other plant?
As for ironwool, I looked at the raws and it is quite interesting. The entire part about yield, stress and all that, is copied straight form iron. About the only change is a bit smaller density and some missing/not_needed_here tags. So it would appear that ironwool possesses the defensive qualities of iron while being a bit lighter.
And it is, interestingly enough, a type of leather, not a cloth or metal. Main template is taken from fleece, which means it acts as leather and you can make normal leather items out of it. You know, cloaks, dresses, backpacks, the works. Though I don't see an option to make armor. Ponies probably don't use it.
And if I understand correctly, it will have the same/similar defensive properties as objects made of iron. Though it is heavier than normal leather/fleece clothing. Density of fleece is 450 while ironwool is 6500 and iron is 7850.
Of course, I might be wrong, since I'm new to modding, so it would be preferable if someone more knowledgeable could confirm/deny my hypothesis.