You have enough questions that you should probably work with an SE or VMConAWS Specialist to present to you. If that's not an option, you might read up on HCX. It's probably going to answer most of your questions.
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If you want to use the UI, then yes HLM is a requirement. If you do not want to configure HLM, you can use PowerCLI to perform the migrations.
It should be noted that vMotion over VPN (Route or Policy based) is not supported. This is due to the nature of the internet, which is what VPN is using as the transport, there is no guarantee on latency or throughput. With that said, you may be successful in performing a vMotion.