For the first question, when you select a mashup in the Composer, if you click on the small drop down menu at the upper left corner of the mashup, you will have a menu entry called "Mashup Parameters". There you can add parameters.
After that, you will create a navigate widget, bound to the second mashup (for which you will also create a parameter) and the widget (not the mashup itself) will expose the parameters of this second mashup. After this, you can bind the parameter for the first mashup to the parameter of the second mashup through the navigation widget.
Another solution can be to use session parameters (available for each session, indiferrent of the mashup). When using those you don't need to modify anything on the mashup.
The session parameters work like this:
-you must go to Subsystems / UserManagementSubsystem and then add a ThingShape that you will create, containing one property.
-this property will appear on the Session tab of the composer (between Data and User) and it will be available for any app that wants to use it on the server.
-you can bind directly to those parameters, it should be straightforward.
If you feel the Session Parameter is too "global" to your liking you can use the Mashup Parameters, or maybe you decide that you don't like modifying the Mashup itself...it really depends on you.
Both can do what you requested.
For the second question, you can create a Service that returns an Infotable and can manually add inside the service some rows with sample data. Then you can bind that Service result to the list. While yes, it may seem not so nice, I think in 5 minutes you can do it and it works :)
I'm not aware at the moment of a way to add manual data to the list in a GUI, only the workaround I mentioned.
Hope it helps,