Every year, you should check the weather wall (wall most affected by the elements). You can use the water bead test, which tests the log surface for absorption. If the water still beads up on the surface, you are good for another season. If it looks like any water was absorbed, you should apply a maintenance coat when the weather is appropriate. Check the other walls as well to determine if a maintenance coat is needed. 

No, you cannot. It will not adhere to the log surface. You should stay with an oil solution or remove the finish to raw wood and start over if you want to switch to a water based solution.

Yes, you can. You should clean the surface prior to application.

If the cracks are 1/4 inch or larger, and are on upper quadrant surface of the log, you should fill those cracks with a textured caulk designed to work with your outside stain.

No. If the cracks are deep, you should fill the crack first with a backer rod or grip strip insulation product, and then finish the crack with the textured caulk material.