On the first question, you'll need an add-on to provide a post-function that can "copy user from picker field into assignee". I'd use the script-runner to do this (it has a built-in script that can do it, and if you decide to do something more clever, you can)
On the second - no, not really. JIRA will pre-fill an empty assignee with a string saying "I'll auto assign this" which the users can select another assignee from if they want, but you can't pre-fill it with the actual selection. The reason for that is you can't know who the assign will be until the user fills in and commits the component field. Once the field is filled of course, you've got an assignee (although you can use a post-function to re-run the auto-assign logic if you want!)
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