1) Since it was garbage time, your coach emptied the bench and played backups for the whole 4th qurter. To ensure that your player gets all 48 minutes, he will have to be set as starter/backup/reserve.
More to the point, the things you'll need to do to get a single player the maximum opportunity to play 48 minutes as a PG:
1. List the trainee as starter, backup and reserve PG
2. Have no more than 8 other players, each of which should either be the starter or backup+reserve at another position.
E.g.
1-1-1
2-6-6
3-7-7
4-8-8
5-9-9
If there are any players not on the lineup but on your bench, they will come in if there is garbage time. You may well be able to get it to work with a player as only the reserve at a position (2-2-6) but I can't personally confirm that and I do know that the lineup above works as well as possible. It still isn't perfect - if one of your non-trainees is shooting FTs when the coach calls for garbage time, his backup will come in for your trainee for a short time. In addition, sometimes the coach just decides that your guy needs to come out. It sucks, but it's not entirely common at least.