Choosing monthly billing means your credit card gets charged once every month. If you choose annual billing, you pay for 12 months at once, but at the price of only 10 months.
Yes, there is a button in your account page with that sole purpose. When you press it you cancel future payments to your subscription, but are still a Pro member until your subscription runs out.
Yes you can. At any point you can upgrade to Unlimited and just pay the difference. If you feel like you need to downgrade back to the Starter plan, you can do that too.
Well yes, as long as you don’t abuse the service. You can download all the icons you need for your projects, but you can’t do abusive things like scrape and download a copy of all icons on the site. Read the Terms of service for more details.
The ones you have already downloaded, you can still use for your projects. But you no longer have access to download any premium icons on Iconfinder.
No. At the moment you can only pay with a valid credit card.