I think home made strawberry jam is always runnier than shop bought. To stop the scum,without adding butter, just allow the jam to cool a little before putting it in the jars,then stir it well while still in the pan until the scum disperses,not sure about the jam maker as I only use a pan. As someone said a bit of apple added at the cooking stage will help it set,not that I use it. Never had any go mouldy,that indicates that it is not kept in a cool dark place,wet jars or that it hasn't got ratio of fruit to sugar right-thats where the boiling comes in to play. Basically you are evaporating the water out of the fruit.
I envy you all enough home grown strawbs to make jam from,I've never had that many plants.
I'm sure someone else will be along to tell you all different to my advise
I've not made any strawb jam for a few years but did used to make loads when the children were small,sometimes a couple of jars made it to the next year & was still good.