You don't need to purchase a theme until you decide to make it live. You can download the draft version and build your site as you please, for as long as you please, until you know you're ready to commit. 

No! You do not need to buy either a plan or the Theme for testing your Shopify site.

You can simply click the “Preview” button to test your site. If you need to share your store with others for testing purposes, you can provide them with your visitor password in Preferences. This allows them to enter and explore your site without the need for it to be live or on a paid plan.

Yes! Until your site is not live you do not need to buy the theme!

Shopify Themes are licensed to use for a single store only! The theme you purchase from the Shopify Theme Store is licensed exclusively for the store you bought it for!

Shopify themes are downloadable, non-returnable goods. We regret that we cannot offer refunds on downloadable products. The reason we cannot refund you directly is because the payment goes to Shopify, and we receive only a commission. For tax reasons, we are unable to provide refunds, regardless of our desire to do so.

You can find the official Refund Policy here:
https://help.shopify.com/en/manual/online-store/themes/adding-themes#buy-a-theme

During the purchase process, before your approval, Charge summary is also highlighted on the authorization site: