It's very easy to do mirror, just you need to add mirror feature and select the right plane, select field bodies to mirror, in your graphic area select the surfaces to be mirrored.
To get a tangancy you must do the below tips :
1- In the picture one, you must made this spline on plane.
2-You must made extruded surface blind of any dimension as shown in picture 2.
3-Now you can made your feature surface (lofted, boundary surface,..), now you can made the tangancy with knit surface enable.
4-See picture 4, how the surface is smooth.