Hi Franz,
Not sure about question 1. but for 2. have you taken a look at the Buffer feature under SPATIAL menu > Buffer Objects
Once you have created the buffer (the buffer can be created in Cosmetic layer and then saved as a new layer if desired), it can then be used to perform a selection from a table of objects which are located within the buffer using SQL Select. Note that objects can be selected from one layer/table at a single time and not multiple tables.
Hope this is somewhat helpful!