I consider the quests Archmage rezo, Cursed Child and Mage Range impossible to do before Freedom Islands, I could be wrong but I doubt. But to get more xp you could do Archmage rezo fights up to Gremlins battle, it should not make you lost too much time but still a little.
The Abandoned Castle quest is certainly to attempt before Freedom Islands. If I remember well Calinda made it but the final fight and a Paladin did it too before Freedom Islands but not sure it was all fights. Myself I didn't but I'll try.
About Turtle Battle there are two tricks:
- The more simple is using Magic Spring + Last Hero + A stack of one unit fast enough to reach Turtle at second round before Turtle attack at second round and despite Turtle slow spell. This solution requires moving close to Turtle, like right to its head (not in front the head) and cast last hero, next round cast magic spring, next round cast last hero and so on. During this time the 4 other stacks, long range will attack the turtle. One caution is always cast a spell when the unit close to turtle has already spend his turn. So you need in your army a stack with higher initiative than turtle but lower than the unit in contact with turtle.
- The other trick is to use 2 stacks of 1 Warrior Thorn each. Move them closer to Turtle. Next round move one close to turtle and the second closer to turtle but close to the other Thorn. Then at each round the turtle kill a Thorn, the other Thorn use the dead body to generate a new Thorn and so on. The turtle will always kill the older Thorn. But you could need use haste at the beginning and Battle Cry if a thorn generated is a hunter instead of a warrior because of initiative problem with the turtle. I'm not sure of those last two points. And for this trick too when a unit go close to turtle at second round it should not be in front of the turtle head.
About 6h15 it's because you forget the animations trick I quoted. You need escape both animations, at travel start and at travel arrival. Then in theory 20mn is enough but myself I let me 30mn to be safer.
EDIT: I'm curious, for this attempt, could you summarize what you've already done before the turtle?