There was indeed an issue with nullable columns which we have fixed.
EventID eq 'SWB2018' and HasOrder eq false looks as a correct filter expression. The only thing I am not 100% sure about is the space it starts with. It is worth trying this just to be on the safe side (no whitespace before EventID):
${event.filter ? event.filter + ' and ' : ''}EventID eq '${this.selectedEvent.EventID}' and HasOrder eq ${!!this.checkboxOrdersOnly.value}
If I set a FilterValue of 0 for a boolean column in my datagrid this will be included in my event.filter?
No. FilterValue is just setting the filtering UI of the DataGrid. The ${event.filter} wont be affected in any way.