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Yes you should use them. They can be taken away from you with 2 min notice. (It doesn't happen a lot in practice though. I have been running a different instance for over a month. AWS doesn't force you if they don't have to)

If you are going to run a long training job, ensure you are creating checkpoints. Be sure to use persistent storage, EBS and ensure that you check the option that it doesn't get deleted if the instance is stopped, so your checkpoint remain in the disk and you can easily restart.

I haven't tried it but prices here are much cheaper. https://vast.ai/#pricing

Yes you can. In Oregon you could eventually get this instance at $9. I say eventually, because of course Spot allocation is not guaranteed. ( And neither is On Demand ...but that is a story for another day)

https://aws.amazon.com/ec2/spot/pricing/

Why not? This is the exact use case of what Spot instances seem to be for. (Not hosting a service, but just calculating something for yourself.)