There can be a huge range in platelet counts, with an above average normal still being completely fine.
Elevated platelets are rarely a concern.
Causes for increase would be due to increased release from the spleen from epinephrine release in the body or during exercise.
Platelets counts can also increase during times of sudden or acute blood loss, iron deficiency, trauma, cancer, or Cushing's disease. For me, it would have to be a persistent platelet count over at least 600,000 before I even bat an eye
Now, the ALT increase is of concern. The ALT is a liver enzyme that can indicate liver damage.
I would consider discussing with your vet doing a abdominal ultrasound to get a better look at the liver, as well as, a bile acids test can be performed to assess the "function" of the liver. Liver enzymes on a blood test only tell about liver damage, not function.