you don’t absolutely need the T1 or T2 scan to compute DTI data. You could also register the PAM50 to your DTI scans without it, if you know the vertebral level of your DTI scans.
Now, if you want to use an anatomical image, you can either use the T1 or the T2 image, depending which one has the best spatial resolution (especially axial resolution). All these aspects are explained in the SCT course.
Regarding the 2nd question: as mentioned above, you do not need to have the T2s data in order to quantify the DTI data.