Ok, after some more research seems that the problem is in the Infopath template.
We couldn't open another template using a XmlFromView while everything went right if we used it directly from Sharepoint. Turned out that the template was made by opening in desing mode an existing one and after some modification saving it with a different name. This left a lot of unused fields hanging inside the schemas and seemed to cause trouble with XmlFromView that responded with an error.
After that, I suspected the same thing was happening here, so I made a simpler template wich worked perfectly. Still trying to figure what's the exact error that makes XmlFormView to leave the controls as readonly, but I'm begining to think it going to be easier and less time comsuming just make the template from scratch.