I am in no way an affiliate for PC Pitstop; however, I'll try to answer most of your questions to the best of my ability using my previous knowledge of PC Pitstop Optimize and other registry cleaners.
Yes, if you reformat your hard drive or reinstall Windows you can reinstall the software at any time. You just need your license key which you can find here after you purchase.
Deleting registry keys can be hazardous although most registry cleaners, including PC Pitstop Optimize if I am not mistaken, allow you to create a back-up file of your registry before you make any changes. You can use this back-up to restore your computer in case something goes wrong. Alternatively, you can find instructions on how to create a manual registry back-up here. However, I doubt PC Pitstop will cause your computer to crash, etc. From my previous experience, it has all went well; It has actually improved my computer's performance and I believe it will do so for yours. Therefore, I recommend that you let it "fix" all problems found; however, I am sure that it would be no problem to post your scanning logs on these forums for more experienced users to take a look at.
Purchasing PC Pitstop Optimize is a one-time only deal, you do not need to make any additional payments. You are entitled to any upgrades or new versions of the product for free.
IMO PC Pitstop Optimize was definitely worth it. I highly recommend you get it, especially if you have no other registry cleaners. You'll be amazed at your computer's faster performance and you'll get far less errors (it even fixed one of my BSOD's lol). My advice would be to go for it.