I don't think you are supposed to use the 50/50 stuff on water supply pipes. That's for heating systems. On water supply I would definetely use lead free.
They have easy flo solder that is supposed be easy to work with. The melting temperature is lower and stay melted over a wider range as I understand it.
I don't see why you are having a problem with the lower lead content solder. It should be the same. If you know how to solder, then you should be able to use any kind of solder. I don't see a reason why one type would work with you and not the other.