Hello,
varying the tempo is not possible at the moment.
Regarding the strings: Cubasis does not have volume automation at the moment, so I would advise you to use the handles of the events. To create a fade-in move the top-left handle of the event, to create a fade-out move the top-right handle. Another possibility would be, if your strings are audio, to create a fade using the sample editor.
If you have any further questions, let me know.
Kind regards,
Frieder