You bring up good points. Currently the redo logs cannot be transported as part of the unplug/plug process.
1) You'll be able to restore DB1 prior to T2 and recover it upto T3. The dropped pdb's datafiles would simply not be part of this recovery session.
2) You'll not be able to restore DB2 prior to T2, since there will be no backups cataloged for the pdb plugged in at time T2. You could unplug the pdb again from DB2, perform the recovery and plug it back in after the recovery is done however.
I think that may be a technical challenge but if that can be considered as an enhancement - it will be a powerful option