1. Bhishma kept Karna with his Vasavi Shakti out of the war for 10 days by his insulting ways. (If Karna had killed Arjuna, the war would have been over. Even if he wouldn't kill the Pandavas he could have killed some "name" warriors like Satyaki or Chekitana.)
2. It looks like all he did was to kill low-level warriors on the Pandava side making sure that the outcome of the war wasn't affected.
3. The most horrific attack in the war was the attack on Bhurishravas to deprive him of his fair victory over Satyaki - surely word must have reached Bhishma about this and he said nothing. He was constantly undermining Duryodhana for being adharmic, whereas Krishna constantly egged the Pandavas on to acts of adharma and even says so at the end of the war.
IMO, all of the above suggests Bhishma was more loyal to the Pandava side than Duryodhana.
Are there any instances where Bhishma proved his worthiness to Duryodhana?
Did Duryodhana ever acknowledge Bhishma's loyalty towards him and the side he was representing?