• Yes, you can remove clusters from Rancher Management and add the back later on.
• You should not be worried. As I explained AKS support for 1.25 and Rancher support for 1.25 may not happen exactly the same day but should always happen within a reasonable timeframe.
• There are no differences between imported AKS clusters and Rancher deployed AKS clusters. Rancher uses the Azure API. You can also import and AKS cluster as a generic cluster but I can't think of any reason to do that (there are no benefits and you loose the value of being able not to just deploy and manage workloads but also changing configs, add/remove nodes, ...)