Mother has type a+ blood, baby had type o+ blood... Blood mixed during birth. Can this result in ABO incompatibility? Baby didn't have any jaundice.

Unlikely. They are both positive. The. Positive represents a protein on the blood cell, thus the mother will not have an allergic reaction to that part, the rh factor protein part. There is a chance for allergies to other parts of the cell making something anti, just not anti -rh. Usually doesn't and not a big deal generally. It is mostly in moms with an rh neg cell that gets infused with rh pos blood.
Unlikely. The main issue we worry about with birth is the mixing of the rhesus groups (+ or -). That is to say that the mother (-) of a positive child (+) is more likely to develop antibodies to the next child and cause problems. The chances of an immunological response with a small amount of blood "mixing" during birth is usually not a cause to worry.
No. A person with o type blood lacks the a and b proteins found on red blood cells (rbcs). If you have type a blood, you have that protein on your rbcs. Abo incompatibility occurs when the mother has o and the baby is either a, b, or ab. The mom can have antibodies to a and/or b, and these antibodies can cross the placenta and attack the baby's rbcs causing hemolysis and jaundice (increase in bili).
Could have been. It could have been and since there was no jaundice i feel that as there was no jaundice the was no incompatibility and the baby's blood was not affected did the baby have anemia it only occurs after birth and not later.