What is the ruling on the prayer of one who found drops of blood mixed with water on his undergarments after istinja' and performed ablution and prayed without paying attention to them?

The drops you mentioned, if you are unsure about the time of their discharge, are presumed to have exited after the prayer.

However, if you are certain that they exited before the prayer, they invalidate ablution, and your prayer, in this case, is void and must be repeated according to the majority of scholars.

As for your prayer with inner garments containing impurity, of which you were unaware, it is valid according to the more correct of the two scholarly opinions.