Since they both discovered where those frozen human remains were, it would make sense if only one of them led them to the spot. Skendus led the way, because this arrangement would be more efficient if he led, and Ortana joined him to keep him company.
Tim was to stay last in line, just to keep the dragon warm, since he too could summon fire of many volumes. He knew enough not to burn the dragon by accident, and he was happy with this arrangement, because he was light enough to ride on the dragon's back as well! A bunch of his childhood fantasies were fulfilled just like that, he was really happy.
Arvena cleared a path for Georgie the dragon to walk on, that was her main job. If they were to make it to their destination, they needed a six foot wide path in front of them, and neither the dragon or Timothy were in a good enough state to carve a path, they couldn't. Arvena was their best hope in this matter, they had to follow her lead.