If I enter different numbers (playing around a bit to see how end result will look for the final refund/payable amount, will the software allow me to?

Yes. Use the navigation menu across the top to jump around. It will appear once you've cleared the About You and Profile sections.

Another nifty feature: Put a bunch of info in a return. Select Tools and then choose Return Manager to jump to the Return Manager and then click the Copy link under the return you just created. Then, change the numbers as you'd like in the copy and compare the copies to each other.

You can always delete your practice returns when you're ready by clicking the little trash can that appears beside them.