@tiagospm
ad 1. WOW, this is really strange. I am changing swimming styles very often during my swim trainings and never experienced such problem…yet. Anyway, are you using default swim sport mode or customized one? If customized, then try to delete it and recreate again. Soft reset could help as well. In case taht you are usgin default swim sport mode, then I have no idea, try soft reset or maybe even hard reset. If none of this helps, than I would get it to Suunto repair service. It could be problem with gyroscop sensor(?)
ad 2. @Egika explained it very pricesely and that is exactly how you have to proceed, when you want to have HR data from chest strap recorded while swimming. Please remember, what Egika said - this is applicable only with Suunto Smart sensor. No any other chest strap will work. OHR on the watch work as well, but data from it are very unreliable while swimming.