make sure the variables passed between models are of the proper data type, and just set the output variable of the manager's checkbox option as the same variable name you're mapping into the workflow model. it should then adopt the same value as was passed in.
as for translating the "True/False" string output of a boolean data type into "Yes/No" as a string, create a throwaway string variable, and when merging that into your email text merge, select the dropdown "Process Defined Field". while that is selected in the dropdown, drag the throwaway text variable onto the text editor, then click the variable object. click the ellipsis on the left to configure the evaluation model. use a true/false component to evaluate the actual checkbox variable, and, using 2 "end" components, have the True path end up at an "end" component that has a static value of "Yes", and the False path leads to an "end" component with a static value of "No". this should get you where you need to be.
process-defined field illustrated below: