Regarding the second bit, you could just change the field to "time" and use the time formating on the inspector. What format do you want the user to enter into the time fields? What is more natural for them?.
Your first part can be solved with a script attached to the radio button (would personally just use any old button). The rehersal join table is just a simple join table or does it have more information? Could you get away with a few lookups in the rehersals table? Just some ideas to help along the process.
Hmmm, "All Performers" means "All performers in the database" or "all performers for this performance?" Leaving that aside for the moment and assuming All performers in your database, you can set up this relationship in order to get a portal listing all performers:
Rehearsals::anyfield X AllPerformers::anyField (AllPerformers is a new table occurrence of Performers.)
Now you can place a portal to AllPerformers on your Rehearsals layout to list all performers.
If you place a button inside the portal's row, it can perform this script to do what you want:
Set Variable [$PerformerID ; Value: AllPerformers::PerformerID]
Set Variable [$RehearsalID ; Value: Rehearsals::RehearsalID]
Go To Layout [Rehearsals by performers]
Set Field [Rehearsals by performers::RehearsalID ; $RehearsalID]
Set Field [Rehearsals by performers::PerformerID ; $PerformerID]
Go To layout [original layout]
You may want to consider an alternative approach. You could choose to use a script to create a record for all performers in the Rehearsals by performers table for each rehearsal record. Then you can add a radio button field to mark the listed performers present or absent and a script trigger on this field can log the time so you can calculate minutes late.
PS. You appear to be using performer names as your performer ID's this is not the optimum way to do this. Names are not always unique, they are subject to change and correcting a misspelled name after related records have been created can be tricky. You can avoid such issues by using an auto-entered serial number for the Performer ID.