If you want to transfer a site, yes. You could have worked on your clients' webflow account but this is not a very good practice. We've asked for an in-between, better solution to collaborate with clients who already have a site or webflow account. I'm sure a solutio will come one day.
Yes but pay attention to the message that will come just before you validate. Some things are going to be reseted, like domain hosting etc.
Absolutely. But a CMS hosting costs 20 when you have a basic plan, it costs 10 if you have a paid plan.
Be aware that transferring a site is... transfering the site, not transfering a copy of the site. Once you click OK, you don't have the site anymore. So you may want to duplicate the site prior to transfer it, so you got to keep a copy for reference and whatever future needs.