[*] How can I manually select records to delete or modify?
Add a Boolean field with a name "Selected", for example. Create a table view, that includes this field, and make the field editable in this table view. Check the "Selected" field in records you want to select. Filter records by the "Selected" field. Now "Delete records" or "Edit Records" commands in table view will be applied to your manually selected records.
[*] How can I make computation using values from different records of the same database?
Currently the only way to use values from different records is through relations. Remember, that you can create a database relation to itself. For example, suppose that you have two records with date field, where one records describes beginning, and the other end of time period. To compute the number of days between the to dates create a relation named "Start", for example. Link the record, containing the end date to the record containing start date. Now you can use both date values in derived field with "days between" operation.