Ideally you should freeze the weights and train the last layers and then unfreeze and train the model. Because initially the weights are randomly assigned and error will be high initially. Unfreezing initially should work although. This post has covered all the cases separately https://forums.fast.ai/t/lesson1-trying-to-understand-fit-one-cylcle-unfreeze-and-max-lr/27963
Not sure of the second question.
Hope this helps
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